We measured the rate of fluid movement across the tracheal epithelium of the dog by an electrical method, modified from that originally described by Wiedner (Rev. Sci. Instrum. 47: 775-6, 1976). The base-line volume flow (Jv) from the submucosa to the mucosa was 44 +/- 23 nl.min-1.cm-2 (mean +/- SE; n = 6). In only three of the tissues was the resting Jv significantly greater than zero. Stimulation with 2 X 10(-3) M aminophylline (a drug that we have shown stimulates Cl secretion under short-circuit conditions) increased Jv in all tissues to a mean of 137 +/- 29 nl.min-1.cm-2 (P < 0.001). We also measured the unidirectional fluxes of Cl with the tissue at its spontaneous electrical potential. During the base-line period, the net flux of Cl was not significantly different from zero. During stimulation with aminophylline, Cl secretion increased to a mean of 1.44 +/- 0.50 microneq.cm-2.h-1. Comparison of the stimulated values of Jv and net Cl secretion suggests that fluid secretion by the trachea is isotonic during stimulation with aminophylline.