Clearance of 14C-labeled methylmethacrylate monomer from the blood was determined in beagles during simulated hip arthroplasty and after intravenous injection of the monomer. Pulmonary function after intravenous administration of the monomer was also studied. Blood clearance was rapid, and the lungs apparently functioned as a major clearing organ. Decreased pulmonary function (decreased Po2, elevated Pco2, and metabolic acidosis) only occurred when the dosage of monomer was more than thirty-five times the amount liberated in humans during total hip replacement.