The objective of this work was to study the effect of small doses of naloxone (Nx) on plasma prolactin (PRL) and testosterone levels, and to correlate hormone changes induced by the opioid antagonist with sexual exhaustion in adult male rabbits. Two groups of 8 adult New Zealand White male rabbits were divided into 2 groups: Group 1 (n=8) was treated with daily intramuscular injections of 0.5 mg of naloxone at 08:00 and at 20:00 hours for 15 days. Group 2 was treated with saline injections. Copulatory behaviour was similar on the first day and during seven days of treatment with Nx in both groups; mounts and copulations averaged 8.1 to 7.21. After 15 days of treatment with Nx, a significant increase in mount/copulation events was observed in naloxone treated rabbits as compared with controls (11.1 vs 7.6) (P<0.001). Mounts and copulations in the Nx treated rabbits was significantly higher than control rabbits (P<0.001) (9.4 vs 7.6) seven days after treatment with Nx was discontinued. During the first week of treatment, plasma testosterone levels in Nx treated rabbits were similar to those of the control group. At the end of the first week of treatment with Nx, there was a continuous increase in testosterone levels and high levels of the androgen were still detectable seven days after naloxone treatment was discontinued. There was a significant overall effect of treatment (P<0.001). It was concluded that endogenous opioids are important modulators of sexual behaviour in rabbits