The effects of acetylcholine (ACh), noradrenaline (NA) and isoprenaline (Isop) on the membrane and mechanical properties of smooth muscle cells of the pig coronary artery were investigated by micro-electrode, double sucrose gap and isometric tension recording methods. (1) The mean membrane potential was -51.4 mV and the membrane was electrically quiescent. Application of outward current pulse generated a graded response. The current-voltage relationship was linear for application of inward current pulses. The length constant of the tissue was 0.67 mm and time constant of the membrane was 290 msec. The tissue possessed cable-like properties. (2) ACh (10(-10) to 10(-5) g/ml.) did not change the membrane potential and membrane resistance, but NA and Isop hyperpolarized the membrane and reduced the membrane resistance. These actions of catecholamines, presumably mediated by beta-adrenergic receptors, were suppressed by propranolol but not phentolamine. (3) The minimum depolarization required to produce the contraction was 4 mV by excess extracellular K concentration and 6 mV by electrical displacement of the membrane potential. The amplitude of the contraction evoked by depolarization in excess K was consistently larger than that by the electrical displacement. The minimum concentration required to produce the contraction induced by ACh was 5 x 10(-9) g/ml. NA and Isop consistently suppressed the contraction evoked by excess extracellular K or by ACh. (4) When the tissue was immersed in Ca-free EGTA solution containing excess K or Ca-free EGTA Krebs solution, the time taken for Ca depletion from the intracellular store site was longer with depolarized membranes than that with polarized membranes. ACh evoked contraction, even after the tissue had lost the ability to produce contraction by excess extracellular K in Ca-free solution. (5) After the tissue had been immersed in Ca-free solution containing excess K for more than 4 hr, or Ca-free Krebs solution for more than 2 hr, application of Ca evoked the contraction. These effects of Ca were suppressed by application of catecholamines. The amplitudes of subsequent ACh-induced contraction in Ca-free excess K were not suppressed, while there was a suppression of the K-induced contraction in Ca-free Krebs solution. (6) It is concluded that the smooth muscle cell of the pig coronary artery possesses muscarinic and beta-adrenergic receptors. The former mainly activates the mechanical response without affecting the surface membrane while the latter modifies both membrane and mechanical properties.