The study was aimed at analyzing the pharmacokinetics of sulphadiazine (SDZ) and trimethoprim (TMP) in geriatric patients and was carried out in two stages. The first stage compared 11 geriatric patients (68-89 years old) and 9 healthy volunteers (21-34 years old). Serum concentration and urine elimination were determined on day 4 after 12-hourly administration of 1 tablet of co-trimazine containing 410 mg sulphadiazine (SDZ) plus 90 mg trimethoprim (TMP). The mean serum concentrations of SDZ were significantly higher in the older subjects than in the volunteers after 1, 8 and 12 h (p less than 0.05, 0.01, 0.01), but were not differentiable after 2 or 4 h. For TMP, the serum levels were higher (p less than 0.05) in the patients only after 8 h. The amounts collected in urine were higher in the normal volunteers for SDZ during the intervals 0-2 h (p less than 0.05), 2-4 h (p less than 0.01), and 4-8 h (p less than 0.01), but not for 8-12 h. The 12-hour recovery was 69% SDZ + acetyl-SDZ recovered in the patients compared to 66.0% in the volunteers. Indistinguishable amounts of TMP were recovered during each interval in the two groups of subjects, 59.2% in the patients compared to 70.0% in the volunteers. The second stage of the study comprised 9 geriatric patients of mean age 83.6 years and creatinine clearance 59.2 ml/min who were followed for 11 days. A steady increase in serum levels was observed. The mean peak serum concentration for SDZ was 16.7 +/- 4.2 mg/l after the first dose and 44.7 +/- 11.3 mg/l on day 10. For acetyl-SDZ, the respective concentrations were 2.1 +/- 0.9 and 7.1 +/- 3.1 mg/l.(ABSTRACT TRUNCATED AT 250 WORDS)