1. Intracellular micro-electrode recordings were made from surface acini of mouse exorbital lacrimal glands placed in a Perspex bath through which oxygenated physiological saline solutions were circulated. Two micro-electrodes were inserted into neighbouring communicating cells. Through one of the electrodes, current pulses could be injected. The cells impaled were stimulated by iontophoresis of acetylcholine (ACh), adrenaline or isoprenaline from an extracellular micropipette. 2. During exposure to standard Krebs solution the resting membrane potential was -42.5 mV +/- 1.2 and the resting input resistance 3.3 Momega +/- 0.3. When the tips of the two intracellular micro-electrodes were more than 100 micrometer apart no electrical coupling between two impaled cells could be detected. At intertip distances below about 80 micrometer coupling was frequently observed. In all such cases the coupling ratio was 1. The resting current-voltage relation was almost linear within the membrane potential range of -30 to -80 mV. 3. During exposure to standard Krebs solution the resting membrane potential was -42.5 mV +/- 1.2 and the resting input resistance 3.3 Momega +/- 0.3. When the tips of the two intracellular micro-electrodes were more than 100 micrometer apart no electrical coupling between two impaled cells could be detected. At intertip distances below about 80 micrometer coupling was frequently observed. In all such cases the coupling ratio was 1. The resting current-voltage relation was almost linear within the membrane potential range of -30 to -80mV. 3. During exposure to standard Krebs solution short iontophoretic pulses of ACh or adrenaline caused fully reversible hyperpolarizations accompanied by marked reduction of surface cell membrane resistance and membrane time constant. The effects of ACh were blocked by atropine (1.4 x 10(-6)M). Iontophoresis of isoprenaline never had any detectable effect on membrane potential or resistance. 4. Applying de- or hyperpolarizing direct currents through one of the two intracellular micro-electrodes the effect of ACh or adrenaline could be observed at different lvels of resting potential. Depolarizing the acinar cell membrane resulted in an enhanced stimulant-evoked hyperpolarization whereas hyperpolarizing the acinar cell membrane resulted in a reduction, and at potentials more negative than -60 mV in a reversal of the stimulant-evoked potential change. The ACh equilibrium potential (EACh) under control conditions was -56.6 mV +/- 1.1 and EAdrenaline was -61.4 mV +/- 1.0. 5. Replacing the control superfusion solution by a Clfree sulphate solution resulted in an immediate shift of EACh towards more negative values. At steady state in the Cl-free solution the resting input resistance was 6.8 Momega +/- 1.3 EACh was -95.9 mV +/- 3.4. 6. Reducing [K]o from the usual 4.7 to 1.0 mM resulted in an immediate marked increase in the amplitude of ACh-evoked hyperpolarization whereas increasing [K]o to 10 mM almost abolished the ACh-evoked potential, but not resistance change. 7...