The female progeny of Angus, Boran and Red Poll bulls mated to Ankole, Boran and Zebu cows were compared. Heifers by Angus and Red Poll sires out of Boran and Zebu cows could be mated at an average age of 23 months with excellent calving percentages, whereas Boran-sired heifers and heifers from Ankole cows were not ready for service at this age. Lactation stress caused heifers suckling calves to lose 4% of body weight over a 7½ month suckling period, while heifers not suckling calves gained 14% in body weight. Heifers by Red Poll sires did not provide more milk for their calves, as measured by early growth rate, than heifers by Angus or Boran sires. The effect of hybrid vigour on mothering ability of F1 dams was at least as great as it was on the early growth of the F1 crossbreds themselves.