Delta9-tetrahydrocannabinol (THC) competes with the endogenous cannabinoids arachidonoyl ethanolamide (anandamide) and 2-arachidonoyl glycerol (2-AG) at cannabinoid receptors. This may cause adaptive changes in the endocannabinoid signaling cascade with possible consequences for the biological functions of the endocannabinoid system. We show that administration of a single oral dose of 20 mg THC to 30 healthy volunteers resulted in higher circulating concentrations of anandamide, 2-AG, palmitoyl ethanolamide, and oleoylethanolamide at 2 and 3 hours after administration as compared with placebo. At 2 hours after THC administration, changes in oleoylethanolamide plasma concentrations from baseline were linearly related to the THC plasma concentrations. In rats, treatment with the CB1/CB2 agonist WIN 55,212 also increased plasma endocannabinoid concentrations. However, this was associated with a decrease of ethanolamide endocannabinoids in specific brain regions including spinal cord, cortex, and hypothalamus; whereas 2-arachidonoyl glycerol increased in the cortex. Thus, administration of THC to human volunteers influenced the concentrations of circulating endocannabinoids, which was mimicked by WIN-55,212 in rats, suggesting that exogenous cannabinoids may lead to changes in the endocannabinoid system that can be detected in plasma.