Plasma insulin and glucose levels have been determined in normal rats, at different times during a 24-hour period. Plasma insulin levels show higher values during daytime, with a maximum at 5 p.m. The levels of plasma glucose were highest from 11 a.m. until 7 p.m., without a clear-cut maximum. Although both periods are rather coincident they do not change in parallel. Present results show that measurements of plasma insulin and glucose levels may give misleading results if control and experimental groups are not killed at the same time of day.