The in vivo response to laser trabeculoplasty (LTP) was evaluated by measuring the incorporation of thymidine into DNA in the nuclei of trabecular meshwork cells. incorporation into DNA was analyzed by light microscopic autoradiography and by scintillation counting of trabecular extracts. Sixteen cats received argon LTP at a power setting of either 0.3 or 1.0 watt with the contralateral eye serving as a control; a 48-h exposure to HT began 1, 5, or 12 days later. The level of HT incorporation into DNA for LTP-treated eyes was significantly higher than controls for the earliest labeling period, but not at the later time points. This pattern was observed for both 0.3-and 1.0-watt treatments. In a second experiment, LTP was performed on six animals; a power setting of 0.3 watt was used in the left eye, and a power setting of 1 watt was used in the right eye. All 12 eyes were radiolabeled for 48 h with HT beginning 1 day after LTP. A small but significant difference in incorporation levels was found between these two power settings. Trabecular cell division may play a role in the therapeutic efficacy of LTP in glaucoma patients.