1. Using a K+-sensitive extracellular electrode, we attempted to determine whether cholinergic stimulation of the simian palm eccrine sweat gland is associated with transient net K+ efflux as in other exocrine glands. 2. When isolated secretory coils placed in a glass capillary were continuously superfused (method A), 32% of total cellular K+ was lost during 3 min of stimulation with methacholine (MCh) followed by K+ reuptake when stimulation was stopped. 3. When secretory coils were stimulated in a small chamber (without continuous superfusion, method C), MCh (5 x 10(-6) M)-induced maximal K+ efflux as determined by the peak level of extracellular K+ concentrations was dose dependent, inhibited by atropine but not altered by a cholinesterase inhibitor, physostigmine (1.3 x 10(-5) M). Thus the peak K+ level was used as a measure of K+ efflux throughout the study. 4. Phenylephrine (10(-4) M) and A23187 (5 x 10(-6) M) also induced K+ efflux but to a lesser extent than did MCh. 5. Ouabain (10(-3) M)-induced K+ loss was 2.4-fold higher than the peak level of MCh-induced K+ efflux. 6. In a Ca2+-free medium with added EGTA, inhibition of K+ efflux was only partial in the first MCh stimulation but progressively increased on repeated stimulation, suggesting that cytoplasmic or membrane Ca2+ not readily accessible to EGTA may be important for K+ efflux. Inhibition of K+ efflux in the Ca2+-free medium was completely reversed on subsequent addition of Ca2+. 7. Five millimolar Ba2+ partially inhibited MCh-induced K+ efflux. 8. 10(-4) M-bumetanide itself caused a small K+ loss and strongly inhibited the subsequent MCh-induced K+ loss. 9. MCh-induced K+ loss was drastically inhibited in the low-Cl- (by replacing with gluconate- or methylsulphate-) or low-Na+ (by replacing with Tris+) medium. 10. K+ efflux occurs predominantly across the basolateral membrane. 11. Vinblastine at 10(-4) M, which completely inhibits sweat secretion (our unpublished results), however, showed no effect on MCh-induced K+ efflux. 12. We conclude that the transient net K+ efflux associated with MCh stimulation constitutes a crucial primary ionic event in cholinergic eccrine sweat secretion as in other exocrine secretory cells.