The effects of marihuana extract and Δ9-tetrahydro-cannabinol on the spontaneous aggressive behavior of the maleadult Siamese fighting fish (Betta splendens) were studied. The drugs were strong suppressors of the aggressive behavior; however, after 8–10 exposures the fishes developed tolerance to the drugs.Continuing the treatment up to 31 days did not induce any further modification in the behavior of the fishes.