A study was developed to measure the effect of three supplementation levels of precalving concentrate, on milk yield of Holstein Friesian heifers, during two seasons (autumn and spring, 1981) at the La Platina Exp. Sta. (INlA, Santiago), in a comple,te randomized designo The following treatments were stablished: T1 = without concentrate, n = 12; T2 = 2 kg of concentrate/heifer/day x 30 days, n = 16; and T3 = 4 kg of concentrate/heifer/day x 30 days, n = 16. After calving, all the animals received during 180 days a basal diet (50% corn silage and 50% alfalfa hay, dry matter basis), plus 6 kg concentrate/heifer/day, mixed with the forage of the diet. When evaluated as a factorial (2 periods x 3 treatments), no interaction was found, so the data was analized as one experimento Milk production to 305 days were 4,553; 4,547; and 4,866 It/heifer, for T1, T2, and T3, respectively (P > 0.05). Body weights (kg) at calving were 526b; 560ab; and 566a, respectively (P < 0.05). The analysis of marginal returns concluded that the most economical diet was T3, with a maximum net return of US$689 in a 305 days lactation period and under a price relation: concentrate/milk = 0.576.