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An experiment was conducted to investigate correlations of litter size and average serum progesterone concentrations during pregnancy with mammary gland growth and development at parturition. Twenty ewes (5, 9, 4, and 2 ewes carrying 0, 1, 2, and 3 lambs, respectively) were used to measure weekly serum progesterone concentration during pregnancy. At parturition, the experimental ewes were slaughtered for determination of mammary gland growth and development at parturition (mammary dry fat-free tissue [DFFT], DNA, RNA, collagen, protein, and glycogen). Correlation of mammary DFFT with litter size and averages serum progesterone concentrations were 0.75 and 0.72, respectively. Litter size or maternal serum progesterone concentrations did not correlate with the mammary DNA concentration. However, litter size or maternal serum progesterone concentrations positively correlated (p < 0.01) with the mammary RNA and protein concentrations, but negatively correlated with the mammary collagen (p < 0.01) and. glycogen (p < 0.05) concentrations. Litter size or maternal serum progesterone positively correlated (p < 0.01) with the total mammary DNA, RNA, collagen, protein and glycogen contents. These results implied that the increased concentrations of progesterone with the increased litter size during pregnancy improved mammary gland growth and development at parturition.