Active Transport of 5,5-Dimethyl-2,4-Oxazolidinedione

5,5-Dimethyl-2,4-oxazolidinedinedione, a substance commonly used to estimate intracellular pH, moves against both a concentration gradient and a hydrogen-ion gradient in the everted gut sac. Furthermore, the value of the flux ratio for this substance under conditions of zero electrochemical potential across the bowel wall unequivocally demonstrates active transport.