Two experiments were undertaken to determine the effects of chlordiazepoxide on intraspecific attack behavior and selected non-aggressive behaviors in male Siamese fighting fish. In Exp. 1, pairs of fish fought while immersed in either 15 mug/ml or 30 mug/ml of chlordiazepoxide, or plain water. The drug groups showed significantly less attack (e.g., biting, jawlocking) than the control group, without noticeable behavioral toxicity. Also, in the drug groups alone, some variants of the copulatory clasp, seen in normal mating, occurred in many pairs. In Exp. 2, individual fish were isolated in one of the same doses or plain water for a period equivalent to that of Exp. 1. These doses produced no changes in measures of arousal, locomotion, and feeding behavior, as compared to the control condition. The drug-related appearance of the intermale mating-like behavior is discussed in terms of a theoretical formulation postulating a mutually inhibitory relationship between sex and aggression in fish.