Live weight, milk yield, fat, protein and lactose percentage of 236 cows and heifers of four breeds of dairy cattle were recorded at 2-week intervals, starting within a week after calving, during the winter and spring of 1972/73. Lactation curves of the form X(n) = Xnb ecn were fitted to each animal's records, where X, b, c are constants, e the base of natural logarithms and X(n) the value of the character at the nth 2-week interval. Correlations between the curves, and between deviations from the curves, showed that the production of milk, fat, protein and lactose was negatively correlated with live-weight change in the long term, but that deviations from the curves were not correlated. There were significant positive correlations between milk yield and size, and between potential compositional quality and size (the values of × in the model). Of the four breeds studied, Friesian, Ayrshire, Guernsey and Jersey, adult Friesian cows required less dietary energy per kg of total solids produced than any other group, according to the principles of the metabolizable energy system of calculating energy requirements. During the period of negative energy balance, an average 10% to 15% of energy output in the form of milk was derived from the mobilization of body reserves.