The existence of cyclical (circadian) variations in lithium toxicity in mice was determined. Mice, standardized to a light-dark cycle, were injected with lithium chloride (940 mg/kg) at one of six times (20:00, 24:00, 4:00, 8:00, 12:00, and 16:00) and subsequently observed for mortality over 28 h. A significant (P less than 0.01) time-of-day effect was found for lithium-induced lethality, with highest lethality following injection at 12:00.