Composite monthly single-day milk samples from the Florida Agricultural Experiment Station dairy herd 1959 to 1974 were analyzed for composition. Data were 22, 972 observations on five dairy breeds, but major statistical analyses were limited to Jerseys and Holsteins. Minimum relative humidity and maximum and minimum temperatures on day of evening sample were associated with 1.6 to 5.6% of variability within lactation (range for two breeds) of milk and milk constituent yields and 1.1 to 16.5% of constituent percentages. Yields of milk and constituents of the Holsteins seemed more sensitive to climatic variation than did Jersey, but Jersey constituent percentages were more sensitive. Yields were affected only slightly by increasing maximum daily temperatures from 8 to 29 degrees C but declined rapidly at greater than 29 degrees C; fat and protein percentages declined from 8 to 37 degrees C, whereas chloride content increased above 21 degrees C. Stage of lactation and pregnancy effects accounted for about 50% of the variability of yields and 3 to 23% of percentages. Effects were detected also for chloride and acidity percentages, specific gravity, and ratios solids-not-fat to fat and protein to fat.