The iontophoretic release of drugs from micropipettes into free (Ringer's) solution was described using an ion-selective microelectrode assay method. This characterization, with a temporal resolution of 20 ms, showed that the equilibrium rate of drug transport was not linearly proportional to release current; the departure from linearity was increased by backing current and the result was demonstrated with analytically derived drug release functions. The general relation between the drug transport rate and release current was independent of the specific drug or pipette resistance; no functional relation was observed that might quantitatively predict this dependence without prior use of the assay. The diffusion coefficients at 25 degrees C in frog Ringer's of the drugs used in this study, all neuromuscular agonists, were determined: all values X 10(6) cm2/s; acetylcholine 6.11 +/- 0.30; carbamylcholine 7.44 +/- 0.34; 3-(m-hydroxyphenyl) prophyltrimethyl ammonium 5.79 +/- 0.13.