Mature weight, maturation rate and lifetime maternal performance, through a maximum of seven calving seasons, were analyzed for 182 cows of Angus (A) and Milking Shorthorn (S) breeds and the two reciprocal crossbred groups. Mature weight and rate of maturation were estimated for the three-parameter growth curve described by Brody. Estimates derived for mature weight (kilograms), both unadjusted and adjusted for cow condition, were, respectively: Angus, 421, 407; Shorthorn, 409, 459; A x S, 448, 455 and S x A, 440, 441 (approximate standard error, 8 kg). Estimates for the maturation rate parameter were, respectively: Angus, 5.8, 6.3; Shorthorn, 6.6, 5.4, A x S, 6.2, 6.1 and S x A, 5.9, 6.1%/month (approximate SE, .3%/month). Heterosis for mature weight was 3 to 7% (28 +/- 8 and 15 +/- 6 kg, respectively). No heterosis was observed for the maturation rate parameter. Birth weight (.048 +/- .025 kg/kg) and weaning weight (.142 +/- .044 kg/kg) of the first calves were positively related with mature size of the dam. Weaning weights of first calves increased with increasing rate of maturation of the dams (3.6 +/- 1.5 kg/percentage/month) while birth weights remained unchanged. Under the culling criteria used in this study, the number of calves produced by a cow in her lifetime decreased (-.007 +/- .003 calves/kg) with increasing mature weight, while average weaning weight of the calves increased (.132 +/- .03 kg/kg). Total weight weaned, years in the herd and weight weaned per year tended to decrease (P greater than .05) with increasing mature weight of dam, but not significantly. Positive heterosis was expressed for all lifetime cow productivity characters, with crossbred cows weaning 22% (230 +/- 57 kg) more calf weight in their lifetime.