Melatonin was measured over 24 hr in the eyestalks of Uca pugilator by means of radioimmunoassay; crabs were acclimatized either to a LD 12:12 photoperiod or constant darkness. A significant peak occurred at 13.00 hr in the LD 12:12 crabs. A photophase peak in melatonin has only been reported in one other species, also a crustacean. In constant darkness, two melatonin peaks occurred, one at 16.00 hr and the other 12 hr later; these results suggest that the melatonin cycle is a true circadian rhythm. HPLC with ultraviolet-visible detection was used to confirm the identity of melatonin immunoactivity. The influence of melatonin on regeneration of the walking legs was also examined: eyestalks were either removed or left intact, and limb bud length was measured every other day for at least 17 days in control and melatonin-treated crabs (60 microg ml(-1) seawater). Melatonin significantly increased the rate of limb regeneration in both eyestalk-intact and eyestalk-removed groups; this is contrary to results of regeneration studies in other phyla, in which similar melatonin concentrations inhibited regeneration.