The absorption of cyclacillin at pH 7.0 by the rat small intestine was investigated using in situ perfusion. At the lowest dose of 95 microgram/ml, the antibiotic disappearance was rapid and followed first-order kinetics, with the disappearance being 85% at 100 min. At the intermediate concentrations of 770 and 1200 microgram/ml, the disappearance after 100 min was 69 and 54%, respectively, and semilogarithmic plots clearly showed convex curvatures. At the highest concentration of 30 mg/ml, cyclacillin disappeared slowly from the perfusate, in an apparent first-order fashion. The disappearance was 26% after 100 min of perfusion and was similar in extent at 5.2 mg/ml. This concentration-time profile was satisfactorily fitted to the simultaneous Michaelis-Menten and first-order kinetic equations. The area under the blood concentration versus time curve (AUC) after a single intraduodenal dose of cyclacillin was almost consistent with the AUC after the equivalent intravenous dose (10 mg/kg). Additional evidence from a pharmacokinetic analysis of steady-state blood concentrations after constant infusion of cyclacillin through the portal vein and the small intestinal lumen indicated that cyclacillin absorption by the rat intestinal tissue at relatively low concentrations (less than 1 mg/ml) followed solely Michaelis-Menten kinetics. Cyclacillin may be transported by certain types of carrier-mediated mechanisms.