It has been demonstrated recently that cephaloridine (Cld) inhibits the tubular reabsorption of filtered carnitine (Carn) in the rabbit kidney. This interaction has suggested that the limited net cell-to-luminal fluid movement of Cld following its secretory transport across the antiluminal membrane might result from a balance of active Cld reabsorption by a Carn carrier at the brush border approximately equal to its secretion into the tubular fluid, rather than the previously proposed luminal membrane block. Studies were done to determine the effects of L-Carn, 750 mg/kg, i.v. on the tubular secretion and cortical concentrations of Cld, infused i.v. at a dose of 28 mg/kg (Carn:Cld molar ratio = 70:1). The fractional excretions of Cld during three consecutive periods of 10 min each, one before and two following the bolus infusion of Carn, were (means +/- SEM): 1.18 +/- 0.14, 1.20 +/- 0.14, and 1.16 +/- 0.11, respectively (N = 6 each; differences NS). Cortex-to-serum concentration ratios of Cld in control and Carn-treated rabbits were 10.43 +/- 0.32 and 10.16 +/- 0.86, respectively (NS). The data provide evidence against the reabsorptive transport of Cld by a Carn carrier, and do not support a model of balanced secretion and reabsorption as the cause of limited clearance despite significant secretory transport of Cld into the tubular cell.