The purpose of the present study was threefold, to examine (a) whether participation in an aerobics class produces an enhancement in the mood state of exercising women, (b) whether any effects persist 24 hours later, and (c) whether exercising in the morning or in the afternoon leads to differential effects. 99 women, aged 19 to 55 years, participated. Of 77 members of a fitness club who formed the treatment group, 28 exercised in the morning and 49 in the afternoon. The control group consisted of 22 nonexercising female clerks. The Profile of Mood States was administered just prior to and immediately after an aerobics class, as well as approximately 24 hours later. Members of the control group completed the Profile at their workplaces, following an identical time pattern. Analysis indicated a significant beneficial effect of exercise at both times of day on all dimensions of mood. 24 hours later, mood scores had not fully regressed to pre-exercise levels. The control group's over-all mood profile was poorer and their responses remained basically unaltered across administrations.