Cimetidine pharmacokinetics was investigated in four groups: Group I with three normals (serum creatinine less than 1.5 mg/dl), Group II with three kidney patients, renal function slightly impaired (serum creatinine greater than 1.8-4.3 mg/dl), Group III with three patients suffering from severe impairment (serum creatinine greater than 4.3 mg/dl) and Group IV with three patients on chronic hemodialysis. All four groups were given cimetidine retard (350 mg tablets Neutronorm retard, Ebewe Arzneimittel GmbH, A-4866 Unterach a.A., Austria). Groups I and II (normals and slightly impaired renal function) received the tablets twice a day with an interval of 12 h (7:00 a.m. and 7:00 p.m.). Groups III and IV (severely impaired renal function and hemodialysis patient were only given one dose a day (7:00 a.m.). The group of normals (I) and the group with slightly impaired renal function (II) had the highest serum cimetidine concentrations one to two h after the morning dose, followed by an exponential decrease. After the evening dose in Group I, the highest concentrations were found after 4 h. Group II had the highest concentrations four to eight h after administration. Compared to the morning dose, the exponential decrease of the serum cimetidine concentration was delayed in onset and slower in phasing out. Furthermore, concentration was higher during the night than during the day.(ABSTRACT TRUNCATED AT 250 WORDS)