Twenty-one compromised neonatal foals hospitalised at the Rural Veterinary Centre (RVC) during 1993 were studied to determine i) serum gentamicin concentrations obtained when gentamicin was administered at 3.3 mg/kg bwt twice daily i.m.; ii) factors which contributed to inter-foal variation in serum gentamicin concentrations achieved and iii) clinical efficacy of gentamicin therapy in foals with confirmed septicaemia. Septicaemia was confirmed in 7 foals with positive blood cultures and suspected in 8 foals with a sepsis score >11. Peak serum concentrations (Ps) were >6 μg/ml in all foals and >8 μg/ml in 60% of foals. Trough serum concentrations (Ts) were <2 μg/ml in all foals. Factors found to produce inter-foal variation in the Ps achieved included age (<24 h; decreased), bodyweight (<38 kg; decreased) and severity of dehydration (8–12% bodyweight; increased). Clinical response was not associated with achievement of Ps >8 μg/ml, but was negatively influenced by the severity of clinical signs of depression. None of the foals in this study developed septic arthritis or pneumonia during or after therapy. No serum biochemical evidence (i.e. elevated serum creatinine concentrations) of gentamicin-induced nephrotoxicity was noted during therapy.