Data are presented on the effect of ethanol on the intestinal absorption and excretion in rats of two beta-lactam antibiotics, cephalexin (CFX) and cefadroxil (CFD). A recirculating perfusion technique within an antibiotic concentration range of 0.5 to 50 mM was used. Ethanol was administered either in an acute form into the intestine or in a chronic form as a 15% drinking solution for 2 months. The results are normalized in relation to the metabolic body weight, intestinal length, and osmotic conditions. Acute ethanol treatment decreases the antibiotic absorption; biliary excretion of CFD is increased, while urinary excretion of CFX is lowered. Chronic treatment shows slight negative effects on the absorption of CFX and CFD. Results are interpreted on the basis of the effect of ethanol on biological membranes. Enhanced urinary excretion after acute ethanol treatment, as well as differences between transport mechanisms, are invoked to explain these effects.