The authors studied the cytotoxic effect of diclofenac sodium, a prostaglandin synthetic inhibitor, on Chang's cultured human conjunctival cells. Diclofenac sodium inhibited cell growth dose-dependently. Although cell growth was interrupted 12 hrs later by one minute of exposure to a 0.1% solution of diclofenac sodium, the cells began to grow again 24 hrs later. Twenty-four hours later, a one-minute exposure to a 0.1% solution of diclofenac sodium revealed no cytotoxic effects electron microscopically. The effect on the cell cycle of exposure to 0.1% diclofenac sodium was studied using a flow cytometer. Twelve hours after exposure to diclofenac sodium, DNA histograms showed a broader G1 peak, and increase in mitotic phase cells and dead cells with a low DNA content on the left of the G1 peak. 24 hrs later, the number of dead cells and DNA synthetic phase cells increased and mitotic cells gradually decreased, almost disappearing within 48 hrs.