Ejaculated mammalian sperm require several hours exposure to secretions in female reproductive tracts, or incubation in appropriate culture medium in vitro, before acquiring the capacity to fertilize eggs. Arachidonylethanolamide (AEA), also known as anandamide, is a novel lipid‐signal molecule that is an endogenous agonist (endocannabinoid) for cannabinoid receptors. We now report that AEA is present in human seminal plasma, mid‐cycle oviductal fluid, and follicular fluid analyzed by high‐performance liquid chromatography/mass spectrometry. Sperm are sequentially exposed to these reproductive fluids as they move from the vagina to the site of fertilization in the oviduct. Specific binding of the potent cannabinoid agonist [³H]CP‐55,940 to human sperm was saturable (K_D 9.71 ± 1.04 nM), suggesting that they express cannabinoid receptors. R‐methanandamide [AM‐356], a potent and metabolically stable AEA analog, and (−)Δ⁹ tetrahydrocannabinol (THC), the major psychoactive constituent of Cannabis, modulated capacitation and fertilizing potential of human sperm in vitro. AM‐356 elicited biphasic effects on the incidence of hyperactivated sperm motility (HA) between 1 and 6 hr of incubation: at (2.5 nM) it inhibited HA, while at (0.25 nM) it stimulated HA. Both AM‐356 and THC inhibited morphological alterations over acrosomal caps between 2 and 6 hr (IC₅₀ 5.9 ± 0.6 pM and 3.5 ± 1.5 nM, respectively). Sperm fertilizing capacity, measured in the Hemizona Assay, was reduced 50% by (1 nM) AM‐356. These findings suggest that AEA‐signaling may regulate sperm functions required for fertilization in human reproductive tracts, and imply that smoking of marijuana could impact these processes. This study has potential medical and public policy ramifications because of the incidence of marijuana abuse by adults in our society, previously documented reproductive effects of marijuana, and the ongoing debate about medicinal use of marijuana and cannabinoids. Mol. Reprod. Dev. 63: 376–387, 2002. © 2002 Wiley‐Liss, Inc.