In syrian hamsters, elevated night-time pineal melatonin levels are quickly reduced to low daytime levels by exposing the animals to light. The purpose of this study was to determine the lowest light intensity capable of causing a large reduction in night-time levels of pineal melatonin in the male hamster. During the dark phase of the light:dark cycle, groups of hamsters were exposed to one of 8 different intensities of white fluorescent light: 5380, 2798, 151, 20.44, 5.38, 1.08, 0.11 and 0.01 lux. For each light intensity, pineals were collected from 8 hamsters each at 2 min before and at 2, 8 and 32 min after the lights were turned on. Pineal melatonin content was determined by radioimmunoassay. Light intensities of 1.08 lux or greater depressed pineal melatonin content significantly (P less than 0.001). Light intensities of 0.11 or 0.01 lux failed to depress pineal melatonin levels. Thus, the apparent threshold for the action of white fluorescent light on hamster pineal melatonin content lies between l.08 and 0.11 lux.