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Heritabilities for and genetic and environmental correlations between uterine capacity, ovulation rate, and body mass (BM) were estimated in mice. Uterine capacity was defined as the number of fetuses (LUC or RUC for left or right side) in one uterine horn for unilaterally ovariectomized females. Ovulation rate (corpora lutea, LCL or RCL for left or right ovary) was measured on the remaining single ovary in these same females. Data on 1,931 mice from four selection populations were used. Left ovulation rate and LUC were measured on 958 animals, and RCL and RUC of another 972 animals were recorded. Genetic and environmental variances and covariances were estimated simultaneously using an animal model with a multiple-trait, derivative-free, restricted maximum- likelihood procedure. Averages for heritability and correlation estimates derived from separate analyses of the selection populations are presented below. Heritability of LUC was higher (.33 ± .06) than that of RUC (.19 ± .02). Heritability of LCL and RCL ranged from .17 ± .03 to .27 ± .06, and heritability for BM was .65 ± .05. The genetic correlation between LUC or RUC and LCL or RCL ranged between .43 ± .29 and .68 ± .05, and between LUC and RUC was .92 ± .05. Body mass had a higher genetic correlation with LCL and RCL (.70 ± .12 and .93 ± .02) than with LUC and RUC (.37 ± .05 and .47 ± .12). Environmental correlations between LCL and LUC and RCL and RUC were .32 ± .09 and .36 ± .05, respectively.