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HPLC separations employing anion-exchange, reverse-phase and paired-ion techniques were developed for the analysts of both ribonucleotides and cyclic nucleotides. The methods for ribonucleotide separation were then optimised and evaluated for the analysis of extracts containing the free nucleotides of rat tissues. The nucleotide patterns of several tissues of the rat were then examined (a) In relation to ageing, over a period of 1 year from birth (b) in rats administered streptozotocin, a diabetogenic agent and (c) in rats administered doses of glucagon adrenaline and glucose. The effects of these compounds were investigated since they are known, like diabetes, to affect carbohydrate metabolism. During ageing, changes in the free nucleotide pattern were slight, but several trends did emerge. For example, the total nucleotide concentration (per 100g of tissue) in several organs declined with age. This was especially noticable with the concentrations of nucleotide-sugars. In streptozotocin induced diabetes the nucleotide pattern of the liver changed markedly, while those of heart and eye were only marginally affected. The nucleotide patterns of muscle and kidney appeared unchanged in the diabetic state. After administration of glucagon, adrenaline and glucose, the nucleotide profiles of liver showed several changes. The changes in nucleotide pattern observed in all the above cases are discussed with reference to biochemical changes which are known to occur in the relevant tissues as a result of the induced metabolic disturbances employed in the present study. Suggestions for further work on the nucleotide patterns of tissues are made. Suggestions are also made for the improvement of techniques for tissue sampling, sample preparation and HPLC analysis. These improvements might be used to facilitate detection of more subtle changes in tissue nucleotide patterns in subsequent studies.