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Effects of herd, year, age, season, and their interactions on milk yield, lactation length, and reproductive efficiency for purebred and high grade Holsteins were determined by analysis of variance of 17,255 lactation records from 48 herds in Mexico. Herds differed in all traits. Herd average milk yield ranged from 2620 to 7670 kg. Years, ages, and seasons were important to milk yield, but these variables had little influence on lactation length, days open, days dry, and calving interval. Most two-, three-, and four-way interactions for milk yield and days in milk were of importance. Main effects and interactions accounted for 37% of total variance in milk yield. Total variance for milk was 1,344,364 kg2. From variance component analysis, percentages of total variance in milk yield attributed to herd, sire, cow, and error effects were 23.8, 2.2, 33.9, and 40.0. Phenotypic correlations among milk yield, days dry, days open, lactation length, and calving interval were positive but significant only for milk and lactation length. Lactation length accounted for about 34% of the variation in milk while days dry, days open, and calving interval contributed less than 4%. For milk yield the estimate of heritability within herd was 11.9% and repeatability 45.1%. Effects of herd, year, age, and season on the performance of Holsteins in Mexico do not differ markedly from temperate areas.