Hydrazine (2 mmol/l) and phenelzine (0.5 mmol/l), which are known to produce hypoglycaemia, inhibit glucose formation from lactate in the perfused guinea-pig liver. The hydrazone formed from pyruvate and phenelzine exerted the same effect at concentrations of only 0.05 mmol/l. It is suggested that the hydrazones are the substances which are effective. All these compounds inhibited pyruvate consumption and decreased CO2 production by the perfused liver which, togeteher with the pattern of hepatic metabolite concentrations, indicate that they diminish pyruvate metabolism. None of them influenced the activities in vitro of pyruvate carboxylase, phosphoenolpyruvate carboxykinase and pyruvate dehydrogenase. The hydrazone compound caused an increase of the ATP/ADP ration at lower concentrations and an opposite effect above 0.5 mmol/l. Nialamide, another hydrazine derivative, also reduced hepatic glucoeogenesis but led to a marked decrease in the hepatic ATP/ADP ratio and liver cell respiration accompanied by a rise in the 3-hydroxybutyrate/acetoacetate ratio.