We studied the effects of the barbiturate thiopental on mouse beta-cell membrane potential and electrical activity under conditions where K+ permeability was stimulated by increases in cytosolic Ca2+. Thiopental (1 mM) antagonized the hyperpolarizing effect of increases in cytosolic Ca2+, increased either by exposure of islets to 50 mM K+ or by reduction of cellular ATP content by 2,4 dinitrophenol.