Human eccrine sweat glands were isolated by shearing and the potential differences across the basolateral membranes determined using bevelled micro-electrodes filled with 4 M potassium acetate. Stable resting potentials of up to -81 mV were recorded. Alterations in external potassium concentration from 1.2 to 100 mM caused the membrane potential to change over a 70 mV range in cells of high resting potential, indicating that the basolateral membrane is largely potassium permeable. Input impedance was determined by constant current injection and found to be in the range 4-80 M omega. On giving a bolus injection of acetylcholine to produce a final concentration of 10(-6)-10(-7) M, four types of response were observed: depolarization, in a proportion of cells with resting potentials of -66 to -80 mV (n = 19), hyperpolarization, in a group of cells with resting potentials of -47 to -70 mV (n = 22), no change, in some cells of -40 to -81 mV resting potential (n = 22) and micro-electrode dislodgement (n = 8). In cells depolarizing to acetylcholine, the depolarization was short-lived and in thirteen cases was followed by a 'rebound' hyperpolarization. Input impedance decreased during depolarization in one-third (n = 5) of the cells in which satisfactory measurement could be made and increased during the final phase of depolarization or during rebound hyperpolarization. In cells hyperpolarizing to acetylcholine, the hyperpolarization was usually accompanied by an increase in input impedance. In ten of the twenty-two cells which showed no change to a first dose of acetylcholine, the agonist was administered at least two more times. In two cells (resting potentials -62 mV, -64 mV) a hyperpolarization was observed whereas in three others (resting potentials -66 mV, -70 mV, -81 mV) depolarization occurred. The effects of acetylcholine, whether depolarizing or hyperpolarizing, were reversibly inhibited by atropine and irreversibly reduced by ouabain. Experiments performed on glands maintained for up to 30 h in supplemented RPMI 1640 tissue culture medium yielded essentially similar results to those performed on freshly isolated glands.