The objective of the present study was to determine the effect of small doses of naloxone on sexual exhaustion in White New Zealand male rabbits. Twelve young and 12 adult male rabbits 6-12 months old and 14-20 months of age, respectively, were selected from a commercial farm. Each male rabbit was housed individually in galvanized cages (90 cm x 60 cm x 40 cm). The rabbits were housed in an open shed exposed to natural photoperiod (12 L 12 D, 19 degrees N). Daily temperature fluctuated through the year from 28 to 16 degrees C. Humidity was 45+/-5%. Water and food (rabbit chow PMI) was supplied ad libitum. After sexual behaviour for each studied group was established, the males were given a 6-day rest, and 3 days before next trial, six males of each group (treated) received a subcutaneous implant of 8 mg of naloxone in a crystalline nitrocellulose pellet formulated to be completely absorbed in 15 days. The remaining six males were sham-treated (control). At the end of the resting period as previously described, the sexual behavior of each group was studied and compared using a Mann-Whitney statistical U-test. The effect of naloxone on sexual behavior was analyzed with a Wilcoxon test for correlated samples. With regard to sexual activity between young and adult rabbits, it was observed that there was a significant difference between groups (P=0.00275, Z=2.8823, adjusted Z=2.99.43) showing that younger rabbits mounted/ejaculated from 9 to 10 females compared with 6 to 8 mounted/ejaculated by older rabbits. When naloxone was administered to both groups, there was a significant difference when comparing sexual behavior before and after administration of naloxone (table first and second trial). Young rabbits treated with naloxone mounted/ejaculated 11-12 females while older rabbits mounted nine females before reaching sexual exhaustion. A significant difference was observed when comparing the number of estrous females that were mounted/ejaculated between groups. Environmental photoperiod and temperature changes were not considered. It was concluded that endogenous opioids are important modulators of behavioral and hormonal interactions related to sexual behavior.