In the present study, we examined the occurrence and potential regulation of endocannabinoid release by cannabinoid CB1 receptors in the rat brain. To this end, we developed a highly sensitive (limit of sensitivity 30-300 amol) new analytical method, combining online brain microdialysis with solid-phase extraction-liquid chromatography-tandem mass spectrometry, which allowed the detection in real time of trace amounts of endocannabinoids in the extracellular fluid. In the hypothalamus, anandamide and 2-arachidonoyl-glycerol release was stimulated following depolarization via local administration of K(+), with or without addition of Ca(2+), or glutamate application. Inhibition of fatty acid amide hydrolase by systemic administration of intraperitoneal (i.p.) URB597 (0.5 mg/kg) induced an increase of anandamide, but not 2-arachidonoyl-glycerol, outflow. The CB1 receptor antagonist rimonabant (10 mg/kg i.p.) increased, whereas the CB1 agonist WIN55,212-2 (2.5 mg/kg i.p.) decreased, anandamide release. Interestingly, the same treatments induced opposite changes in 2-arachidonoyl-glycerol release. At a dose of 3 mg/kg i.p., which by itself did not affect endocannabinoid release, rimonabant fully antagonized the effect of WIN55,212-2 (2.5 mg/kg i.p.). Taken together, these results suggest that CB1 receptors are able to control the local release of endocannabinoids in the hypothalamus via a feedback mechanism and strengthen the view that anandamide and 2-arachidonoyl-glycerol have distinct physiological roles.