There is increasing evidence to suggest that several effects of the formamidine pesticide amitraz (AMZ) in mammals are mediated by its interaction with alpha 2-adrenoceptors. AMZ has been shown to inhibit the binding of [3H]clonidine, a specific ligand for alpha 2-adrenoceptors to mouse brain in vitro and after administration in vivo. In the present study we have further investigated and characterized the effects of acute and chronic administration of AMZ on brain alpha 2-adrenoceptors in mice. AMZ caused a dose-dependent inhibition of [3H]clonidine binding. This inhibition was long-lasting (more than 48 h) following a relatively high dose of AMZ (75 mg/kg), while it was of short duration (2 h) following low doses (7.5 and 12.5 mg/kg). The time course of inhibition of [3H]clonidine binding was correlated with the plasma levels of AMZ and/or its active metabolites, measured with a novel radioreceptor binding technique. The alteration of [3H]clonidine binding was due to a decrease in alpha 2-adrenoceptor affinity, with no change in the density of binding sites, and was reversible in vitro upon repeated washing of the membrane preparation. Repeated administration of 7.5 mg/kg or 12.5 mg/kg AMZ, to yield a total dose of 75 mg/kg, showed no evidence of a cumulative effect on brain alpha 2-adrenoceptors
