The usefulness of argon laser trabeculoplasty (ALT) in the treatment of some patients with glaucoma has been established; yet, the mechanism of ALT is not understood. The hypothesis that the mechanism involves trabecular cell division was examined. Autoradiographic assessment of tritiated thymidine incorporation into trabecular cell DNA in a human corneoscleral explant organ culture system demonstrated a basal level of DNA replication that increased by 180% in the two days immediately after ALT. When labeling was initiated five days after ALT, DNA replication declined to 70% of the control level. The autoradiographic findings were confirmed using biochemical evaluation of tritiated thymidine incorporation into acid-precipitable macromolecules (DNA) in excised trabecular meshwork. One biologic response of the trabecular meshwork after laser trabeculoplasty is a change in the level of ongoing trabecular cell division.