In the year ending September 1977, 242 British Friesian cows were recorded as having produced more than 10 000 kg in a 305-day lactation. The model y(n) = anbe−cn was fitted to the monthly test day yields of each lactation (a, b, c are constants, e the base of natural logarithms and y(n) the average daily yield in week n). The model explained 87·7% of total variation in the loge y(ri), and the mean constants were 31·8, 0·3160 and 0·0373. Seasonality of production was confined mainly to a spring peak of about 10% above an approximately constant level of production during the rest of the year. These lactations generally peaked later, were more persistent and showed less response to seasonal variation than average.