The present study reports on the effects of in vivo single and multiple isoproterenol (ISP) injections on the proliferative response of subsequently explanted mouse submandibular glands. Explants were cultured in Waymouth’s medium supplemented with 10% fetal bovine serum, and were harvested after 1 2,4 or 6 days in culture. 24 h prior to harvest, the explants were fed medium containing [3H] thymidine. At the time of harvest, the explants were fixed in situ and processed for light-microscopic radioautography. The number of labeled nuclei and the total number of nuclei were counted, and labeling indices were calculated for the control and ISP-treated explant sections. The results show that the ISP-treated explants had more cells actively synthesizing DNA than did the corresponding control explants after 1 day in culture. In addition, after 2 days in culture, the explants from mice that had received a single dose of ISP incorporated more [3H] thymidine than the corresponding control explants. These results suggest that the in vivo ISP stimulus initiated a hyperplastic response in vivo that was completed during the 1 st or 2nd day in vitr