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Environmental factors affecting milk yield were assessed for 279 lactations made by 193 Hereford cows in two herds during 1968 and 1969. Years and sex of calf were significant; whereas, herds and service sires within years were not statistically significant. Dams nursing female calves gave about 56 kg more milk over a 205-day lactation. This result is at variance with some reports in the literature. Of the regression effects, quadratic responses in milk yield due to age of dam and calving date were found to be significant. Peak yield was observed at 8.4 years. Significant linear effects were found for calf birth weight and cow weight, but both of these effects were small in comparison to age of dam and calving date effects. Examination of 205-day calf weight revealed significant effects of years, sires, milk production, calving date, calf birth weight and cow weight; whereas, age of dam effects were not significant. On a within herd-sex-year basis, approximately 60% of the variance in 205-day weight could be attributed to the direct influence of the dam's milk yield. Thus, it was concluded that effects due to age of dam are expressed primarily through differential milk production. Percent protein, percent solids-not-fat and percent fat had small nonsignificant correlations with 205-day weight. Therefore, it appears that milk quantity rather than milk quality was more important in its influence on 205-day weight.