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In recent years, there have been many reports of the beneficial effects derived from the taking of large, i.e. gram, doses of ascorbic acid. These reports have stressed the harmless nature of the vitamin, due largely to its water-solubility. However, these reports have been countered by a smaller number describing adverse effects of large doses of ascorbic acid. Such reports have been given considerable publicity in spite of the fact that few of the effects have been systematically investigated. When gram doses of ascorbic acid are consumed, it is probably no longer possible to consider the substance as a vitamin for the doses are pharmacological and the substance itself becomes a drug and like other drugs becomes involved in the detoxification system of the liver. The activity of the drug metabolising enzymes is known to be reduced by ascorbic acid deficiency. The bulk of the work described in this thesis involves a series of experiments designed to investigate the effects and mechanisms of action of large doses of ascorbic acid over periods of 4 and 28 days on the hepatic drug metabolising enzymes, using the guinea pig as an animal model, since this species, like man, is unable to synthesise the vitamin. The relative importance of the effects found is discussed together with possible mechanisms of action. In the doses given, which correspond approximately to 4.5g/day in man, ascorbic acid had an inhibitory effect on the mixed function oxidase system, which was not. extended to a generalised effect on the endoplasmic reticulum or the liver cell, but was characterised by altered haem synthesis. The activities of cytochromes P450 and b[5] and the concentrations of microsomal haem were decreased as was hydroxy-lation of type I substrates. However, type II substrates were unaffected, suggesting that specific species of cytochrome P450 may have been involved. The concentrations of the phospholipid, phosphatidylcholine and its precursor, phosphatidylethanolamine, were also decreased. A dose dependent relationship was apparent, with decrease in the concentrations of the cytochromes first occurring with dose levels of 200mg/day, with exacerbation of the effect with 300mg/day. Phase II metabolism of drugs was also affected, with the activity of 1-naphthol glucuronyl transferase showing a decrease. Another membrane-bound enzyme, glucose-6-phosphate, was unaffected, as was the cytosolic enzyme, glucose-6-phosphate dehydrogenase. The extent of lipid peroxidation also remained unchanged. Activity of the rate-limiting enzyme in haem synthesis, delta-amino laevulinic acid synthetase, was significantly increased, whereas the activity of the degradatory enzyme, haem oxygenase, was decreased, suggesting an attempt to conserve haem. The possible involvement of transition metals, such as copper, zinc and iron, was investigated but no significant trends were seen in the concentrations of these metals in either the serum or liver of control and treated animals. Ascorbic acid also appears to be capable of augmenting the action of the natural superoxide anion scavenger, superoxide dismutase. This finding is perhaps significant since such ions may be involved in the hydroxylation of xenobiotics, perhaps as precursors of the active oxygen species associated with cytochrome P450. Both deficiency and large quantities of ascorbic acid appear capable, therefore, of exerting a similar inhibitory effect on the hepatic mixed function oxidase system of the guinea pig. Hence results of these studies suggest that large doses of ascorbic acid can no longer be considered to be totally innocuous and. care should be taken when prescribing large doses of the vitamin to at least some groups of the population. Finally, it should not be forgotten that other adverse effects, besides those affecting the drug metabolising system, have been reported in the literature, including increased excretion of uric acid and oxalate. Although reports of these effects have been used to counter the claim that the use of large doses of ascorbic acid is without risk, this argument is no longer valid since these effects have recently been shown to be negligible. However, the results presented in this thesis provide strong new evidence of a possible adverse effect of large doses of the vitamin.