The effects of melatonin administered chronically on forced-swim test and open-field test behaviours were examined in male and female rats. The forced-swim test has been shown to be sensitive to all major classes of antidepressants and evidence indicates that melatonin possesses putative antidepressive properties. Male and female Long-Evans rats received either a regimen of chronic administration of melatonin or the control condition for 14 days via the drinking water. On day 15, each animal was individually introduced into a swim chamber, and was scored for 15 min on the duration of swimming, struggling, and immobility. After 24 h, each animal was again tested in the forced-swim test for 10 min. On day 18, all animals were tested in the open-field test apparatus for 5 min. Results revealed that females consistently showed higher activity levels than males in the forced-swim and open-field tests. Melatonin significantly increased struggling in males on day 15, but failed to do so in females. Also, whereas melatonin-treated females showed higher levels of behavioural immobility during their first exposure to the forced-swim test, this effect was prevented upon a second exposure. In both males and females, melatonin decreased swimming in the forced-swim test while increasing open-field ambulatory behaviour. Therefore, it is unlikely that melatonin's mechanism of action is a general inhibitory effect on motor activity. Taken together, the results suggest that the effects of melatonin treatment on forced-swim test behaviours are sex- and test-dependent.