The autoradiography of diffusible, hydrophilic solutes presents special problems in localization of the labeled solute under study. We present studies of the movement of 14C-labeled urea, and 14C- and 3H-labeled sucrose across the isolated urinary bladder of the toad, a vasopressin-sensitive epithelium, using a technique that avoids exposure to water throughout all processing steps and minimizes error caused by isotope scatter. We have shown a significant increase in 14C urea entry into epithelial cells following vasopressin, and a significant decrease following phloretin, an agent that selectively blocks vasopressin-stimulated urea transport. The autoradiographic technique confirms the luminal site of action of phloretin. Studies of 14C and 3H sucrose labeling show that this molecule is virtually excluded from the cell. The current method of grain counting is capable of yielding reliable information in studies of epithelial transport.