If circadian variations in tolerance to drugs have been often demonstrated, such observations about heavy metals are still very scarce. The present study proposes to determine if circadian variations occur in the tolerance of Mice to single lethal dose of mercuric chloride. 360 female Mice, kept in cages on a 8.00 - 20.00 L/D cycle, receive I.P. a single mercuric chloride injection at different concentrations (100 to 300 X 10(-6)) at different hours in the day (8.00 - 14.00 - 20.00 or 2.00). Death number is determined each during 10 consecutive days. For the highest doses causing death (from 200 x 10(-6)), the mortality percentage is a function of the time of the mercury administration. The metal is the most toxic at 20.00 (LD50: 4,14 +/-0,47) mg/kg) and least at 2.00 (LD50: 5,54 +/- 0,18 mg/kg). Moreover, death occurs later at 2.00 than at other times. So a susceptibility to mercuric chloride in Mice clearly appears.