Sexual behaviour in rats can be used to predict putative effects on human sexual behaviour. Anecdotic reports exist, that the beta-lactamase inhibitor, clavulanic acid exerts sexual stimulating activities in monkeys. To characterize these pro-sexual activities, clavulanic acid was tested in three doses and compared to one dose of a sexually inhibitory dose of the selective serotonin reuptake inhibitor, paroxetine, in sexually-experienced male rats, selected for a moderate level of sexual performance in a standard 30-min test with an oestrus female. After acute administration, clavulanic acid had minor sexual stimulating effects at the highest dose in the number of intromissions and in the first ejaculation series. After sub-chronic 7-days treatment, clavulanic acid increased the number of ejaculations at all three doses and reduced the number of intromissions in the 1st series at the highest dose. After chronic 14 days treatment, a similar but stronger pro-sexual profile was observed. The sexual side effects of paroxetine were as expected, including slight sexual inhibitory effects after acute administration, but somewhat stronger overall inhibitory effects after 7 and 14-days pretreatment, particularly notable in the decreasing number of animals contributing to the 2nd ejaculation series, which was even stronger after 14-days treatment. One week after cessation of treatment, the paroxetine group had completely recovered, whereas the highest dose-group of clavulanic acid still showed some pro-sexual effects. This remarkable pro-sexual activity of clavulanic acid cannot readily be explained by its mechanism of action as a beta-lactamase inhibitor but could be due to unexpected central activity of the compound.