Insulin increases active sodium transport by the toad urinary bladder within 15 min, an effect which persists for an least 20 h. In tissues pre-treated with inhibitors of transcription or translation, sodium transport briefly increases after insulin addition but returns to basal levels within 60-90 min. We have studied the effects of insulin on the incorporation of radioactive amino acids into mucosal cell proteins. Insulin had no detectable effect on the uptake of amino acids nor on their incorporation into total protein; however, using a dual label technique, we found that insulin increases the incorporation of amino acids into specific soluble and plasma membrane proteins of the granular mucosal cell. The time course and dose-response relationship of the induction of protein synthesis by insulin suggest that these proteins may play a role in the sustained elevation of sodium transport by insulin.