Daily rhythmicity of serum testosterone concentration in the mature male laboratory rat was examined under various lighting schedules. In rats living in a standard light cycle (12-h light, 12-h dark; lights on at 0600 h), a trimodal rhythm was predominant, with elevations near 0200, 1200, and 1800 h. This pattern was reasonably stable in seven different studies, despite differences in experimental design, method of blood collection, anesthesia, and whether individual rats were sampled once or repeatedly, and was found both in groups of animals and in individuals, including a study using 40-day-old rats. In constant illumination, the pattern was disrupted, but in constant darkness the trimodal pattern was maintained, indicating that the rhythm is endogenous. In a reversed light cycle (12-h dark, 12-h light; lights on at 1800 h), the "midday" elevation was reversed; in an altered light cycle (12-h dark, 12-h light; lights on at 2300 h), the time of the "midday" elevation was shifted. Serum testosterone concentration was higher during the light phase than the dark phase, and was higher in constant light than in constant darkness. A seasonal shift in the daily rhythmicity of serum testosterone concentration is suggested. The trimodal rhythmicity contrasts with the circadian rhythmicity of other hormones. Its functional role in the life of the animal is unknown.