Genetic parameters were estimated for milk and fat yield traits in Iranian Holstein cattl, using data collected during years between 1992 and 1999 in Animal Breeding Center, Iran. Estimates were carried out through Restricted Maximum Likelihood procedures, using univariate, repeated-records, and multivariate Animal Models. The number of records were 60589, 31367 and 14265 for lactations 1, 2 and 3, respectively. Estimates of heritability from univariate analysis in lactation 1, 2 and 3 were 0.27, 0.23 and 0.14 for milk yield and 0.23, 0.21 and 0.14 for fat yield, respectively. The heritability and repeatability estimates from repeated-record analysis were 0.22 and 0.47 for milk yield, and 0.22 and 0.43 for fat yield, respectively. Pooled estimates of genetic parameters from bivariate analysis for the first and second lactations were similar to the corresponding univariate estimates. However for third lactation, these estimates were greater than the results for univariate analysis. Additive genetic correlations between performances of lactation 1, 2 and 3 for each yield trait were very high, ranging from 0.85 (between lactation 1 and 2 for fat yield) to 0.98 (between lactation 2 and 3 for milk yield). The corresponding estimates for phenotypic correlations were positive and moderate whereas for residual correlations were positive and low.
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