Summary Records from a large farm in the Zona da Mata in Brazil were studied to learn about conditions which affect milk production under the penkeeping system. Production varied significantly from year to year. Part of this is ascribed to an upward trend with time, but the statistical significance of that trend is not wholly assured. Some of the yearly means deviated rather widely from that trend. Season of year had significant but rather small effects on the daily yield. The higher yields were toward the middle and end of the rainy season, while the lower ones were late in the dry season. Production reached a maximum some time around 40 to 50 days after calving. Thereafter it declined in almost a straight line. The sex ratio showed a slight but statistically significant excess of males. The year-to-year deviations of the sex ratio from the general mean were not statistically significant. Heritability of individual differences in milk yield under these conditions seems moderately high.