In patients with normal hepatic and renal function, between 30 and 60 percent of administered ceftriaxone is eliminated by nonrenal (biliary) mechanisms. Substantial nonrenal elimination reduces the need for dose adjustments in mild and moderate renal impairment. Minor increases in the biologic half-life (12 hours versus normal of 8 hours) of ceftriaxone have been seen in (functionally) anephric patients with normal extrarenal clearance mechanisms. Anephric patients with decreased nonrenal elimination (additional liver damage) showed a greater increase in biologic half-lives (greater than 15 hours). In patients with various degrees of liver insufficiency (alcoholic fatty liver and cirrhosis with and without ascites), only those with ascites showed significant changes in total drug clearance and volume of distribution. However, these changes in patients with ascites were such that they did not demonstrate significantly different biologic half-lives (9.7 hours versus normal of 8 hours). Simulations of observed concentration versus time data support a physiologic disposition model whereby ceftriaxone, like other cephalosporins, distributes only in plasma and in the extravascular-extracellular (interstitial) fluid and ceftriaxone is saturably bound to albumin in both spaces. All observations in normal subjects and patients were in good agreement with the physiologic disposition model predictions. The consequences of the nonlinear binding behavior of ceftriaxone are such that they favor the administration of ceftriaxone in a large single dose rather than in divided doses. No major drug accumulation is expected in patients with renal or hepatic insufficiency, but anephric patients with a decrease of more than 80 percent in nonrenal elimination will require dose adjustments.