Sulphadiazine (SDZ) and trimethoprim (TMP) combined into co-trimazine has a half-life of 10 h in the adult and therefore the use of 1 dose/day has been introduced in the treatment of patients with urinary tract infections (UTI). To investigate the pharmacokinetics of cotrimazine given once daily in infants and children, 14 patients aged 3-49 months were given 0.3-0.4 ml/kg of suspension containing 41 mg SDZ and 9 mg TMP/ml. In all patients the peak plasma levels of SDZ at steady state were greater than 15 mg/l and after 12 h mostly greater than 10 (range 7-22) mg/l. The corresponding levels of TMP were 0.5 and 0.1 (range 0.11-0.48) mg/l. The concentrations in urine at 24 h were in the children about 40 mg/l of SDZ and 10 mg/l of TMP and in the infants 35 and 2 mg/l respectively. Therefore, in the treatment of infants and children with UTI, 1 dose of co-trimazine/day should give adequate plasma and urine levels.