Six adult domestic shorthair obese cats were given 3-mg/kg gentamicin sulfate by rapid i.v. and by s.c. injection in a cross-over design. The plasma concentration time data were analyzed using statistical moment theory with no assumption of a specific compartmental model. Means ± SD for the half-life, which was calculated from the terminal slope of the log concentration-time curve, were 1.37 ± 0.24 and 1.24 ± 0.22 h following i.v. and s.c. injection, respectively. The apparent volume of distribution at steady state was 118.55 ± 19.83 ml/kg, and total body clearance was 1.07 ± 0.25 ml/kg/min. Bioavailability was 83.58 ± 14.83% after s.c. administration. The calculated s.c. dose in obese cats to produce an average steady-state concentration of 4 μg/ml is 2.5 mg/kg every 8 h compared to 3 mg/kg in normal-weight cats.