Genotypic and phenotypic variances and covariances for milk and fat yields of three equally-spaced intervals in first lactation, days 1 to 90, 91 to 180, and 181 to 270, were estimated from 26,523 records of initial progeny-test daughters of 2,086 Holstein sires. Henderson's method 3 was used with a model that included fixed herd-years, fixed age-month of freshening, and random sire and residual effects. Heritabilities for milk yields were .21, .21, and .13 for the first, second, and third 90 days in lactation and for fat yields were .19, .16, and .10. Heritabilities for milk and fat yields over the entire 270 days postpartum were .22 and .22. Genetic correlations among partial yields were high and ranged from .74 to .99 for milk and .86 to .99 for fat. Genotypic and phenotypic variances were used to predict breeding values for trimester milk yield by multiple trait, mixed model procedures for 3,797 Holstein artificial insemination sires from 283,900 first-lactation milk records of their daughters during the first 90, second 90, and third 90 days in lactation. Correlations among proofs for the three trimesters ranged from .85 to .94. Correlation between the sum of three trimester multiple trait proofs and proof for milk yield for 270 days of lactation from single trait analysis was .99. Correlations among proofs indicate sires may rank differently for milk yield in the three periods.