Anandamide (20:3, n - 6) (homo-gamma-linolenylethanolamide) and anandamide (22:4, n - 6) (7,10,13,16-docosatetraenylethanolamide) are known to be present in porcine brain and to undergo specific binding to cannabinoid binding sites. We have now shown that both compounds inhibit the electrically evoked twitch response of the mouse isolated vas deferens (IC50 = 99.3 and 95.5 nM respectively) indicating that they also have the ability to elicit a response. As electrically evoked contractions of the mouse vas deferens are also inhibited by the endogenous cannabinoid, anandamide (20:4, n - 6) (arachidonylethanolamide; IC50 = 52.7 nM), and by other cannabinoids, we conclude that anandamide (20:3, n - 6) and (22:4, n - 6), may, together with anandamide (20:4, n - 6), serve as endogenous cannabinoid receptor agonists. This conclusion is supported by our other main finding, that vasa deferentia show tolerance to the inhibitory effects of anandamide (20:3, n - 6) and anandamide (22:4, n - 6) when obtained from mice subjected to a delta 9-tetrahydrocannabinol pretreatment that is known to induce cannabinoid tolerance.