Acute administration of certain cannabinoids, including Δ9-tetrahydrocannabinol (Δ9-THC), resulted in elevated levels of plasma corticosterone in mice. The rank order potency of these cannabinoids is the same as others have reported using behavioral tests. The maximally effective dose of Δ9-THC (30 mg/kg) in this test also increased the amount of 3H-tryptophan found in the brains of mice given an intravenous injection of 3H-tryptophan 10 min prior to decapitation. This effect was associated with an increase in the amount of 3H-5-hydroxytryptamine synthesized during the pulse period. Adrenalectomy was found to inhibit these effects of Δ9-THC. The possibility that corticosterone may mediate the effects of Δ9-THC on tryptophan disposition and metabolism is discussed.