The capacity of pupil dilation to affect light-induced plasma melatonin suppression was tested by exposing human subjects with freely constricting or pharmacologically dilated pupils to either 50 (n = 6), 100 (n = 8), or 200 lux (n = 5) of white light presented over the entire visual field. Pupil dilation significantly enhanced low level white light-induced melatonin suppression over that elicited with freely constricting pupils. Although 100 and 200 lux white light exposures resulted in significant melatonin suppression over control (no light) conditions, the effects of 50 lux were not strong enough to demonstrate statistically significant suppression with six subjects. Linear regression did not reveal a systematic relationship between theoretical retinal illuminance in Trolands and magnitude of melatonin suppression. These results suggest that pupil diameter may be a factor in the effectiveness of light stimuli used to shift circadian rhythms or to treat seasonal depression or sleep disorders.