During a 5-year study of lambing behaviour involving almost 1900 births by grazing ewes, the incidence of lambing peaked during the 5-h period 0900-1400 hours, when 28% of ewes lambed. Significant numbers of births occurred throughout the day, and so effective supervision of lambing would not be possible if attendance was restricted to daylight hours. The birth process was clearly delayed when the unlambed sheep were moved slowly from 1 paddock to another. The apparent tendency for ewes to lamb in clusters was shown to be largely the outcome of the exponential distribution expected for the intervals between consecutive, randomly occurring events, but a small significant excess of short intervals above those expected indicates that environmental factors may operate in the synchronous triggering of parturition.