Ceftriaxone (CFTX), a third-generation cephalosporin, has occasionally been reported to produce biliary sludge composed of its calcium salt. We performed studies in guinea pigs to (1) investigate the hepatic route of CFTX excretion, (2) determine ceftriaione's effects on bile flow and composition, and (3) quantify the solubility and metastability of the calcium salt as a function of administered dose. Our results show that even at high doses ceftriaxone has only minimal effects on bile flow and biliary electrolyte secretion, either alone or in combination with bile salt (taurocholate) infusion. A significant increase in total calcium concentration was observed without change in free Ca2+ concentration, this is compatible with formation of a soluble calcium salt of ceftriaxone, as previously demonstrated in vitro. Ion products of Ca2+ and ceftriaxone as high as 3.5 times the solubility product constant without crystal formation were observed, confirming the presence of a metastable state for the calcium salt of ceftriaxone in the living animal. Biliary excretion of ceftriaxone inhibited excretion of indocyanine green, suggesting that ceftriaxone and indocyanine green share a common anionic excretory pathway in this species.