The endogenous cannabinoid anandamide produces cannabimimetic effects similar to those produced by delta9-tetrahydrocannabinol (delta9-THC), but has a much shorter duration of action due to its rapid metabolism to arachidonic acid and polar metabolites via action of fatty acid amide hydrolase (FAAH). Our earlier observations that anandamide's effects persisted after brain levels of anandamide itself had substantially dropped prompted us to examine the influence of the irreversible amidase inhibitor, phenylmethyl sulfonyl fluoride (PMSF), on the brain levels and pharmacological effects of anandamide. As shown previously, pretreatment with PMSF resulted in a leftward shift of the anandamide dose effect curves for antinociception and hypothermia in male mice. Brain and plasma levels of anandamide, arachidonic acid and polar metabolites peaked at 1 min after i.v. injection with 3H-anandamide and remained high at 5 min post-injection, with levels falling sharply thereafter. Pretreatment with PMSF (30 mg/kg, i.p.) prior to an injection of 1 or 10 mg/kg 3H-anandamide resulted 5 min later in enhanced brain levels of anandamide compared to those obtained with 3H-anandamide plus vehicle injection. Levels of arachidonic acid and polar metabolites in brain were not significantly increased. The clear correspondence between brain levels of anandamide following pretreatment with PMSF and pharmacological activity suggests that this parent compound is responsible for the antinociception and hypothermia that occurred 5 min after injection. These results further suggest that metabolite contribution to anandamide's effects, if any, would occur primarily at later times.