A single dose of hydrazine (3 mg.kg-1 i.p.) caused hepatic accumulation of triglycerides and depletion of ATP in rats after 9 h. Repeated exposure of rats to hydrazine (approximately equal to 2.5 mg.kg-1 per day) for 10 days resulted in depletion of hepatic reduced glutathione (GSH) and triglycerides. Repeated exposure to hydrazine also caused a significant (time dependent) induction of p-nitrophenol hydroxylase (NPH) activity together with changes in other hepatic microsomal enzymes. These included 7-pentoxyresorufin O-deethylase (PROD) and 7-ethoxyresorufin O-de ethylase (EROD) activity, total cytochrome P450, cytochrome b5 and cytochrome P450 reductase activity. Repeated exposure to lower levels of hydrazine (approximately equal to 0.250 mg.kg-1 per day) caused no significant hepatic biochemical or microsomal changes after 5 or 10 days except for an increase in NPH activity (17%) and liver ATP (15%) after 5 days.