We report the pharmacokinetic parameters of ceftriaxone in 11 patients on hemodialysis with end-stage renal disease (ESRD; creatinine clearance < 5 ml/ min/1.73 m²). The patients were studied during the interdialysis period and during 4h of hemodialysis. The mean age was 53.4 years. After the administration of 1 g of ceftriazone during a constant intravenous infusion over a 30-min period, t_½ was 16.6 h, β was 0.0418 ± 0.0106 h^-1, V_D was 14.5 ± 3.0 liters/1.73 m² and Cl_p was 0.40 ± 0.05 liters/h for the interdialysis period. Hemodialysis started 24 h after the infusion. The initial plasma ceftriaxone concentration was 68.6 ± 10.8 μg/ml. This value dropped to 40.4 ± 4.7 μg/ml at the end of the 4th hour, indicating a significant 41% decay in blood levels during hemodialysis (p < 0.001). The t_½ decreased to 4.88 h, k_el rose to 0.142 ± 0.0250 h^-1 and Cl_p increased to 1.73 ± 0.44 liters/h. All values were highly significantly different (p < 0.001) from those during the interdialysis period. The plasma ceftriaxone concentration of 40.4 ± 4.7 μg/ml at the end of hemodialysis was well within the therapeutic range of the drug. We conclude that ceftriaxone has a moderated increase in t_½ in patients with ESRD. Ceftriaxone is significantly dialyz-able, however, the plasma concentrations are in the therapeutic range by the end of a 4-hour hemodialysis, 28 h after the administration of the drug. We propose that 1 g given intravenously before each hemodialysis will be sufficient to keep the patient’s plasma concentrations within the therapeutic range until the next hemodialysis. If the interdialysis period is longer than 48 h an extra dose should be administered.