We have investigated the binding of 123I-labeled N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methy l-1 H-pyrazole-3-carboxamide (AM251), an analog of the cannabinoid receptor antagonist SR141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-me thyl-1 H-pyrazole-3-carboxamide] in the mouse brain. Following intravenous injection, the peak whole-brain uptake of about 1% of the administered activity occurred at about 2 h. By 8 h radioactivity in brain had declined to about half its peak value. High-performance liquid chromatographic analysis showed that > 70% of radioactivity extracted from brain at 2 h was still present as [123I]AM251. Co-injection of SR141716A inhibited the in vivo brain binding of [123I]AM251 dose dependently. At 2 mg/kg, the highest dose that could be tested, inhibition was 50% at 2 h post-administration. The ED50 value calculated assuming that 2 mg/kg gave near-maximal inhibition was about 0.1 mg/kg. In contrast to the brain, radioactivity in other major organs (blood, liver, kidney, heart and lung) was little affected by SR141716A. The regional binding of [123I]AM251 in the brain was consistent with the published distribution of cannabinoid receptors in rat brain, in that the order was hippocampus, striatum > cerebellum > brain stem. delta 9-Tetrahydrocannabinol co-administered intravenously at 10 mg/kg, a dose which induced catalepsy and decreased locomotor activity, decreased the 2 h brain uptake of [123I]AM251 by 10%, but this was not significant (P = 0.08). In in vitro binding assays with mouse hippocampal membranes, tetrahydrocannabinol inhibited binding of [123I]AM251 with an IC50 value of about 700 nM, compared with about 0.2 nM for SR141716A.