1. An analysis has been made of milk records from 3109 cattle of the six main dairy breeds in England and Wales.
2. The correlation between measurements on separate lactations declined as the time interval between the lactations increased. The average correlation was 0·38 for 70 days' milk yield and 0·48 and 0·55 respectively for milk yield and fat content in the 305-day lactation.
3. The heritabilities of the three measurements measured by daughter-dam regression were found for heifer lactations to be 0·36, 0·43 and 0·43 respectively. On second lactations, they were 0·09, 0·24 and 0·42.
4. As a consequence of the low heritability of the second lactation yield, the average yield of the first four lactations proved to have no more value as a prediction of a cow's breeding merit for yield than did her heifer yield.
5. In agreement with other workers, we found that 70-days yield was an excellent guide to the total lactation (r ~ 0·80), and that it could be used as a guide to early selection.
6. There was some indication that the correlation between yield and fat content in the same lactation was more strongly negative in heifers and in the Channel Island breeds. The overall genetic correlation on first lactations was found to be − 0·66 ± 0·20, rather more strongly negative than has been found previously.