The intraperitoneal injection of clenbuterol, a beta-2 adrenoceptor agonist which readily penetrates into the brain, modified the copulatory pattern of male rats. In sexually vigorous rats, the acute administration of clenbuterol at the dose of 0.1 or 1 mg/kg reduced the number of mounts and/or intromissions and increased the postejaculation interval, while the repeated administration (7 days) of a dose of 0.1 mg/kg/day had no effect. In sexually sluggish rats, both the acute (0.01 or 0.1 mg/kg) and repeated (0.1 mg/kg/day x 7 days) administration of clenbuterol increased the percent of animals achieving ejaculation and succeeding in initiating a new copulatory series within the respective cutoff times, and reduced mount, intromission and ejaculation latencies, as well as the postejaculation interval. These data show that clenbuterol negatively affects the copulatory behavior of sexually vigorous male rats, but improves that of sexually sluggish ones, and provide further evidence that central beta-receptor activation produces behavioral effects.