The pharmacokinetics of a range of substituted benzyl -cyclopropane-l-carboxylates topically applied in acetone to adult mustard beetles, Phaedon cochleariae, and in Sirius mineral oil to larval Spodoptera littoralis were investigated with particular reference to tissue binding and distribution. A set of pyrethroids with a wide range of binding properties was applied to adult mustard beetles. The form of the pharmacokinetic profiles was obtained by exhaustive soxhlet extraction of the tissues. Binding varied with the physicochemical properties of the compounds. Two phases of binding were identified; rapid binding to cuticle occured within seconds of topical application, followed by a slower binding which proceeded to a maximum after several hours. When cypermethrin was applied to larvae of S. littoralis, two similar phases were observed; rapid binding which took place over the first hour was followed by a slower binding which continued at a constant rate for up to 72 hours after dosing. Increasing the viscosity of the carrier oil reduced the rate of penetration of cypermethrin into larvae of S. littoralis. The tissue concentration of cypermethrin when equilibrated throughout the larval tissues was related to the ratio of tissue solids to tissue water. This suggests that the distribution of insecticide in the tissue is determined by partition processes. All of the tissues without exception reached steady state within one hour of dosing and the tissue levels maintained thereafter. However, this pattern was not observed for the gut contents, where cypermethrin levels reached a peak after six hours. Thereafter cypermethrin disappeared - from the gut, presumably as material was eliminated or degraded. The gut appears to be the only important site of loss of cypermethrin from Spodoptera larvae. The toxicological implications of these results are discussed.