IN order to investigate further the relationship of cellular metabolism to active ion transport, we have correlated carbon dioxide production with sodium transport by the urinary bladder of the toad, Bufo marinus, in vitro. The toad bladder was mounted so as to separate the two halves of a glass chamber, and each side was bathed with a phosphate Ringer's solution (Na 115.3, K 1.88, Ca 1.78, Cl 114.9, HPO4 4.0, and H2PO4 0.1 m.equiv./l., brought to pH. 6.5 with iso-osmotic HCl). Carbon dioxide-free air was bubbled through the solutions at a rate of 40 ml./min. The effluent carbon dioxide was trapped in alkali and measured by a method described previously1. The half-time for efflux of carbon dioxide from the chamber was 4 min. Each period of collection began after equilibration for at least 20 min and lasted 1 h. Sodium transport was monitored by the short-circuit method2, using an automatic instrument designed and manufactured for this purpose by Mechrolab, Inc., Mountain View, California.