Mechanisms of pyrethroid resistance in cypermethrin-selected tobacco budworms, Heliothis virescens were studied using a pharmacokinetic assay and radiolabelled cypermethrin. Selection of larvae and adults resulted in increased levels of resistance, reduced rates of penetration and increased excretion of cypermethrin metabolites. Although both life stages expressed the same mechanisms, development of resistance was faster in colonies selected as adults rather than larvae. Results from in vitro assays using model substrates for mixed-function oxidase, esterase and glutathione S-transferase activities were not correlated with the increased metabolism measured in pharmacokinetic assays suggesting that activity toward these model substrates did not reflect that of enzymes involved in cypermethrin metabolism and resistance.