a total of 1578 lactations of 493 cows were analyzed between 1990 and 2001. lactations were grouped according to calving season (autumn and spring), month of calving within season (autumn: march, april, may; spring: july, august, september), cow age, number of calvings and lactation year. accumulated production up to 305 days was considered; shorter lactations were not adjusted in order to detect environmental effects on the production period. monthly milk production was adjusted to 30 days and standardized at 4% fat content. variables evaluated were milk production, production and concentration of protein and fat, lactation length and persistency. independent variables were analyzed estimating average minimum squares. with autumn calvings, milk production, fat production and lactation persistency were 5,293 kg, 205 kg and 72.1%, respectively, being greater (p ≤ 0.05) than spring calvings (5,114 kg, 198 kg and 66.2%) with no difference in protein production. during the first half of lactation, milk production was lower with autumn calvings and the opposite was observed in the second half of lactation, indicating in both cases an underutilization of production potential of the cows for nutritional reasons. lactation yield increased with earlier calvings in both seasons (5,600 vs. 5,440 kg in march and may with autumn lactations; and 5,303 vs. 4,980 kg in july and september with spring lactations). milk protein content was greater (p ≤ 0.05) with spring lactations compared to autumn lactations (3.23 vs. 3.19%, respectively) with no differences in milk fat content (3.72 vs. 3.71%).