A series of ponds in the Wear Valley in northern England, ranging in altitude from 46 to 838 m, were visited regularly over a four-year-period, and a record was kept of the dates on which spawn from the Common frog, Rana temporaria, was first seen in each of the ponds. On average. spawning was delayed by six days for every 100 m increase in altitude. The date of spawning at each of the ponds was fairly consistent from year to year; only in 1974 was the date of spawning noticeably different from other years. A theory was proposed which could account for the variation in the date of spawning with altitude and the annual variation in the date of spawning, observed in the present study. It is proposed that frogs from nothern England become sensitive to environmental temperature in the middle of February and emerge in the spring after experiencing a temperature-sum of 106 degree(C)-days. The onset of temperature sensitivity is probably initiated by a circannual endogenous rhythm. The time at which the rhythm switches on the temperature-sensing mechanism in frogs probably varies in different populations, and may well be locally adapted. For spawning to follow emergence, a certain minimum water temperature may be required. In the present study, most frogs in the wild spawned when the water temperature in the ponds was above 7°C, but frogs could spawn at 3·1°C. Alternative theories for the control of emergence and spawning are also discussed.