Environmental light cycles are the dominant synchronizers of circadian rhythms in the field, and artificial light cycles and pulses are the major tools used in the laboratory to analyse properties of circadian systems. It is therefore surprising that few studies have analysed the physical parameters of light stimuli that affect circadian rhythms. There have previously been no spectral sensitivity measurements for phase shifting the circadian rhythms of mammals and only two preliminary reports on the wavelength dependence of this response exist. Using the magnitude of phase shift caused by a single 15-min pulse of monochromatic light given 6 h after activity onset, we have now characterized the spectral sensitivity of the photoreceptors responsible for phase shifting the locomotor rhythm of the hamster (Mesocricetus auratus). The sensitivity curve for this response has a maximum near 500 nm and is similar to the absorption spectrum for rhodopsin. Although the spectral sensitivity is consistent with a rhodopsin-based photopigment, two features of the photoreceptive system that mediates entrainment are unusual: the threshold of the response is high, especially for a predominantly rod retina like that of the hamster, and the reciprocal relationship between intensity and duration holds for extremely long durations (up to 45 min). These results suggest that the photoreceptive system mediating entrainment is markedly different from that involved in visual image formation.