Insulin is essential for the accumulation of rat casein messenger RNA (mRNA) in the presence of glucocorticoid and prolactin. The accumulation of certain mRNA's in other tissues has also been linked to insulin action. The present study shows that the accumulation effect on the 25,000 molecular weight rat casein mRNA does not reflect stabilization of the transcript by insulin. Rather, insulin is essential for its synthesis in the presence of glucocorticoid and prolactin.