We investigated the numbers of DNA-synthesizing cells in mouse submandibular glands (SMGs) during chronic isoproterenol (IPR) administration, using an immunohistochemical method with anti-bromodeoxyuridine (BrdU) antibodies. Adult and immature female mice were injected with IPR daily or every other day up to a total of seven injections. Whereas hypertrophic enlargement of acinar cells continued with repeated IPR injections, induction of DNA synthesis was transient. With one IPR injection, more than 50% of the acinar cells were induced to synthesize DNA, but after five daily injections the numbers of cells in the DNA synthetic phase decreased and returned to control values. This pattern was basically the same for immature mice and for adult mice stimulated by IPR every other day. The responsiveness of the SMGs of immature mice to IPR was somewhat lower than that of adults. Prolongation of the interval between injections of IPR, compared to daily injections, did not cause sustained induction of DNA synthesis. These data suggest that the hypertrophic enlargement and the induction of DNA synthesis by IPR are mediated through different intracellular pathways after the stimulation of beta-adrenergic receptors, and that the regulatory mechanism for the induction of DNA synthesis is complex.