Abstract:The effects of a 1‐hr light pulse on the timing of the circadian rhythm in the blood plasma concentration of melatonin were documented in Soay rams. Groups of 5 to 6 animals were transferred from short days (LD 8: 16) to constant dim red light (DD) for 6 days, and were exposed to a 1‐hr light pulse at one of 16 different times throughout 24 hr on day 3. Blood samples were collected hourly for 30 hr before (day 2–3) and after the light pulse (day 5–6), and the plasma concentrations of melatonin were measured by radioimmunoassay. The animals were allocated to experimental groups based on the circadian time (CT) when the light pulse was given using two hourly blocks through the circadian day; the onset of enhanced melatonin secretion (melatonin peak) was designated as CT 12. Under DD there was a clearly defined plasma melatonin rhythm in all animals. The mean duration of the melatonin peak was 13.24 ± 0.16 hr (n = 91) and the mean period between the onset of successive melatonin peaks was 23.55 ± 0.10 hr (n = 21). The effect of the 1‐hr light pulse on the time of onset of the melatonin peak varied significantly with the circadian time when the light pulse was given (ANOVA,P= 0.031). Light‐induced significant (pre‐ vs post‐pulse onset, Students t‐test,P<0.05) phase delays in the onset of the melatonin peak in the early subjective day at CT 2.5 hrs (mean ø: ‐1.9 hr), and in the early subjective night at CT 12.5 and 14.5 (mean ø: ‐2.0 hrs), but not at other times. The light pulse never induced significant phase advances. The effects of the light pulse on the offset of plasma melatonin peak did not vary significantly with the time of the light pulse (ANOVA,P= 0.780), although significant differences in the pre‐ and post‐pulse offset occurred at CT 14.5 and 18.5 (mean ø: ‐1.5 hr). The differential changes in the onset and the offset of the melatonin peak resulted in changes in the duration of the peak (maximum difference between means: 3.8 hr). The results indicate that entrainment occurs under natural 24 hr LD cycles when light impinges on the early subjective night and induces a net phase delay, thus extending the period of the melatonin rhythm to 24 hr. This causes a close phase relationship between the end of the light period and the onset of the melatonin peak as occurs in sheep under natural cycles. The results are also consistent with a multiple oscillator governing melatonin secretion, and that differential entrainment of the component oscillators by light affects th