We studied pharmacokinetics of chloramphenicol in 9 neonates having a mean gestational age of 31.2 ± 1.9 weeks (mean ± SEM). The studied dose was the final dose of treatment in 8 of these and the first dose in 2 of these. 1 neonate was studied twice. Concentrations of chloramphenicol and its 3-monosuccinate and 1-monosuccinate esters were measured in serum by high performance liquid chromatography. Apparent total body clearance of chloramphenicol correlated with postnatal age (r = 0.81, p < 0.01). Mean apparent clearance was 1.1 ml · min^-1 · kg^-1. Serum concentrations of succinate esters were below assay sensitivity after 6 h postdose. Factors leading to excessive chloramphenicol concentrations (> 25.0 mg/l) were evaluated in another 44 newborns. Instability of the patient’s clinical condition was an important cause of excessive serum concentrations during ongoing therapy.