Netilmicin, a half-synthetic aminoglycoside, was given to 22 patients with urinary-tract infections at a dose of 2 mg/kg body weight twice daily intramuscularly for 7--9 days. Pharmacokinetics (in ten patients), efficacy and tolerance to the drug were analysed. Mean serum peak concentrations on the first and last days of treatment, was 8.7 microgram/ml at 45 min after injection, 9.5 microgram/ml after 60 min. After 12 hours the level had fallen to 0.8 microgram/ml and 1.2 microgram/ml, respectively. Mean half-life was 3.08 hours for the first and 3.57 hours for the last day. The corresponding mean values for plasma clearance were 53.6 ml/min and 42.4 ml/min, respectively. Within 12 hours an average of 77.6% and 78.0%, respectively, of the netilmicin dose were eliminated in the urine. The causative organism was eliminated in 12 of 16 patients with chronic or reccurent and in all six with acute urinary infections. There were two re-infections and two failures. Superinfection did not occur. Local and general tolerance was good and there were no signs of renal or inner-ear toxicity.