The pharmacokinetic parameters of sulfamethoxazole and trimethoprim were measured in 12 newborn infants of less than 3 days postnatal age, following a single or repeated intravenous injection. The mean half-lives for SMZ and TMP were 16.5 hours and 19.0 hours, respectively. The mean volumes of distribution were SMZ 0.48 L/kg and TMP 2.7 L/kg. The mean drug clearances were SMZ 0.65 ml/minute and TMP 3.31 ml/minute. From these data the recommended loading dose is SMZ 10 mg/kg with TMP 3 mg/kg, and the maintenance dose is SMZ 3 mg/kg with TMP 1 mg/kg twice daily. No adverse side effects were seen during treatment, and the bilirubin-binding capacity of albumin were not changed at therapeutic concentrations of the antimicrobial drugs.