Mean daily milk yield, fat concentration and protein concentration in the milk of 579 303 British Friesian, 6 608 Shorthorn, 27 374 Ayrshire, 19 529 Jersey and 19 760 Guernsey cattle were recorded monthly from August 1978 to July 1979 inclusive, and classified by breed, parity, stage of lactation, and five geographical areas according to the location of the herd. Parameters of the function Y(n) = anbecn were estimated for each breed/parity/area group for yield, fat concentration and protein concentration (b and c describe the shape of the curve, a is a scalar, n the week of lactation and e the base of natural logarithms). Systematic deviations from the function were associated with the season of production, reflecting the spring and summer flush of milk yield. Fat and protein concentrations also exhibited seasonal variations from the underlying curve. After correction for the seasonal fluctuation in production, there was no variation in total yield, fat concentration or protein concentration associated with the month of calving.