There were 282 cows that completed one or more lactations on grazing alone (T1); or grazing plus supplements of molasses (T2), ground corn (T3), high concentrate feeding (T4), limited concentrate feeding (T5), and urea + molasses (T6). Supplements for T2, T3, T5, and T6 were fed at the rate of 1 lb/2 lb of milk in excess of 22 lb/day and for T4 at the rate of 1 lb of concentrate per 2 1b of milk irrespective of level of production. Stocking rate was one cow per acre for all groups on highly fertilized pastures of star, pangola or guineagrasses in a humid area of Puerto Rico. During the dry period all cows were on grazing alone. Calvings were for the period October 1969 to February 1974. Temperature conditions and rainfall were adequate for growth of grass throughout the year. Average milk and fat yields, using age-month of calving adjusted records, were 6,912 and 245; 8,005 and 275; 8,080 and 284; 9,861 and 335; 9,300 and 315; and 8,424 and 286 1b for T1, T2, T3, T4, T5, and T6, respectively. Treatment effects were significant (P < .05) for lactation length, milk and fat yields, and final body weight but were not significant for fat percent, gain in body weight, or measures of breeding efficiency. There was no association between initial body weight and treatments. Breeding efficiency, determined as time from parturition to first estrus, and calving interval were below average on grazing alone (T1) and high supplement (T4). When milk yields were adjusted to expected yield per cow per year, the gains over grazing alone were 20.2, 21.4, 42.0, 39.0, and 33.3% for T2, T3, T4, T5, and T6, respectively. From these results medium, but not high, levels of supplementary feeding of lactating cows on high quality tropical grass pastures are economically feasible.