A single-dose kinetic study of oral timolol, 20 mg, was undertaken in 3 groups of volunteers with varying degrees of renal function--(1) 10 normal subjects (N); (2) 9 patients with moderate chronic renal insufficiency (MCRI; C cr, 20 to 50 ml/min); (3) 4 patients with end-stage renal disease (ESRD)--to assess the need for dosage modification as renal function diminishes. There were borderline statistical differences in absorption between groups. The mean peak concentration (C max) was 84.3 +/- 44.8 ng/ml at 0.8 +/- 0.4 hr for N and 87.1 +/- 22.8 ng/ml at 1.7 +/- 1.2 hr (p, NS) for MCRI. N and MCRI mean half-lives (5.2 +/- 2.6 hr and 4.0 +/- 1.2 hr) were not statistically different. Salivary levels correlated with plasma levels in 3 N and 1 MCRI patient. Group differences in blood pressure and pulse response to timolol seems to reflect differences present at baseline with percent change from baseline identical for the two groups except at 12 to 24 hr. Administration of timolol on an interdialysis day revealed similar kinetic and physiologic response in the normal and the MCRI group. During dialysis, timolol, 20 mg, induced significant hypotension and bradycardia.