The objectives were to examine the rate of synthesis of casein mRNA transcripts in bovine mammary tissue at different hormonal states and to study the effects of hormonal stimuli (insulin, hydrocortisone, and prolactin) on the accumulation of casein mRNA and on the rate of protein secretion by epithelial cells from bovine mammary tissues. Total cytoplasmic RNA was extracted from mammary tissues of cows obtained by biopsy (8 mo pregnant) and upon slaughter (lactating). The relative specific activities of cytoplasmic mRNA for alpha s1-, alpha s2-, beta-, and kappa-casein were about 3.2, 4.6, 3.3, and 4.5-fold higher in tissues of lactating cows than in those of 8 mo pregnant cows. Mammary alveolar epithelial cells retained hormone-inducible milk protein gene expression for total milk protein gene expression for total milk protein secretion and for alpha s1- and beta-casein messages. Prolactin, even in the absence of insulin and hydrocortisone, induced significant amounts of milk protein mRNA. Hydrocortisone in the presence of prolactin amplified the lactogenic effects on mammary epithelium. Maximal induction of beta-casein mRNA and protein secretion occurred when all three hormones were present.