Pharmacokinetics and metabolism of two carbamate insecticides, carbaryl and landrin, in the rat.

1. A pharmacokinetic model for distribution of carbaryl and landrin (0-5 mg/kg) in male rat has been derived. Distribution of both compounds is adequately described in terms of two kinetically distinct compartments equilibrating slowly or rapidly with the blood circulatory system. The role of the slowly equilibrating tissue compartment is particularly important for carbaryl and as a result this carbamate has a longer half-life (77 min) than landrin (42 min). 2. Less than 1% of the carbamate was excreted unchanged. The kinetics of metabolism have been studied by decline in plasma carbamate concentrations, urinary excretion, and production of 14CO2 from the 14C-carbamyl group. Enterohepatic circulation of carbamate metabolites prolongs the duration of radioactivity in the body. 3. Comparison of different routes of administration shows that oral or hepatic portal administration results in lower plasma carbamate levels than those achieved by jugular vein injection, due to a liver first-pass effect.