To assess the effect of glaucoma and timolol on tear secretion, basal tear turnover was measured with fluorophotometry in 13 open-angle glaucoma patients not using any ophthalmic medication, 24 patients using timolol medication daily, and 41 healthy control subjects. Basal tear turnover is defined as the tear turnover at the lowest level of reflex lacrimation possible under physiologic conditions. Tear turnover was calculated from the decay of the tear fluorescence after instillation of fluorescein. Minimal influence of reflex lacrimation was obtained by instilling 1 microliter of 2% fluorescein without touching the eye and by discarding measurements performed in the first 5 min. Minimization was confirmed by a monophasic decay of tear fluorescence. The values of patients who used timolol and those who did not use timolol were significantly lower than those of healthy control subjects (mean values in percent/minute +/- standard deviation: 10.1 +/- 3.2, 12.3 +/- 4.1, and 15.6 +/- 5.4, respectively; Student's t-test: P < 0.02). The values of patients who used timolol were significantly lower compared to those of patients who did not use timolol (P = 0.03). The tear film break up time values of patients who used timolol were significantly shorter than those of patients who did not use timolol and healthy control subjects (Fisher exact test: P < 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)