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Local blood flow of the hypothalamus (HBF) was measured by the hydrogen clearance method in anaesthetized dogs. The average value for HBF in normal controls was 0.64 +/- 0.05 ml/g/min, which compares favourably with values available in the literature. During haemorrhagic hypotension induced by a modified Wiggers method there occurred a marked reduction of HBF to 52% of the initial control value at a mean arterial blood pressure (MABP) of 55--60 mm Hg and to 44% at 35--40 mm Hg. Prevention of concomitant extracellular acidosis by infusion of sodium bicarbonate solution during the hypotensive period resulted in a significantly smaller decrease in HBF as compared to an untreated control group, and a significant protection of HBF was also found at 55--60 mm Hg MABP in the bicarbonate treated group, but not in controls treated with physiologic saline. Hypothalamic tissue hypoxia was inevitable in untreated bled animals due to insufficient circulatory transport, while evidence suggested that the metabolism might have remained unaltered in the group protected against acidosis during haemorrhage. Analysis of the control of local hypothalamic blood flow revealed a significant correlation (r = 0.7026, p less than 0.001) between HBF and arterial blood pH in severe hypotension which is outside the autoregulatory blood pressure domain.