Parathion undergoes enzymatic oxidation by hepatic cytochrome P-450 (CYP450) enzymes to the active metabolite paraoxon. Consequently, alterations in CYP450-dependent oxidation may affect the pharmacokinetics and pharmacodynamics of drugs that are metabolized in the liver. The CYP3A family is known to be responsible for the majority of cyclosporine metabolism. The aim of the present study was to assess the disposition kinetics of cyclosporine during subchronic parathion exposure. Male Wistar rats were administered either water or two different doses of parathion (1/100 LD50, 1/25 LD50; LD50 = 14 mg/kg) by gavage for 6 wk. Subsequently, rats in each experimental group received a single oral dose of cyclosporine (10 mg/kg), and serial blood samples were drawn from the carotid artery over a period of 48 h. Pharmacokinetic analysis showed that parathion increased the blood cyclosporine concentration twofold as evidenced by AUC (area under the curve), half life (t 1/2) and peak plasma concentration (Cmax). This may be due to inhibition of cyclosporine metabolism, an interaction that may be of clinical relevance in immunosuppression therapy.