Different groups of Helix pomatia were exposed to short light pulses (1 or 2 hours) during a long dark period (16 or 14 hours) of a 24-hour cycle of light and dark. The effect of the light pulses on the hibernation of the snails was shown to depend on the circadian time the pulses were introduced. Some of these light pulses reduced the hibernation. In other experiments groups of snails were exposed to 12-hour cycles or 24-hour cycles of equal periods of light and dark. Hibernation was reduced by the former as compared to the latter. These results show that Helix pomatia exhibits photoperiodic control of hibernation by a discontinuous or cyclic mechanism of time measurement.