A double-blind study of pindolol eye drops 0.5% was carried out on 24 patients with primary open-angle glaucoma. Pressures were taken once weekly during three 3-week periods. In one of the periods placebo was given in both eyes, during another pindolol was given in one eye, and during another it was given in the other eye. A drop in pressure averaging 2.79% was found in eyes receiving pindolol compared with the period when they were receiving saline. Substantial placebo effects were found when in-trial pressures were compared with pretrial, but the placebo effects were avoided in the analysis of the pindolol effect. Blood pressure and pulse rates were not significantly affected. There was slight evidence of pindolol in one eye affecting the contralateral eye but little evidence of persistence of hypotensive effects after cessation of treatment.