1. The action of adrenaline (in the presence of propranolol; 3 x 10(-6) M), adenosine triphosphate (ATP) and carbachol on guinea-pig taenia caeci, and the interaction between these agonists, was studied by measuring changes in membrane potential using the sucrose-gap method in quiescent preparations at 22 degrees C.2. A sustained hyperpolarization was caused by addition of adrenaline (3 x 10(-6) M) and by applying adenosine triphosphate (ATP; 4 x 10(-4) M) for 5 min in Krebs solution. In calcium-free medium containing EGTA (0.4 mM) and high magnesium (6.2 mM), both the alpha-agonist and ATP caused a transient hyperpolarization which passed off within 5 min, although the agonist was still present.3. The transient hyperpolarization evoked by these agonists in the absence of calcium could be evoked only once. The response was restored after exposure to high calcium, (40 mM for 2 s, or 10 mM for 30 s). The maximum amplitudes of the hyperpolarization caused by adrenaline or ATP after exposure to high calcium (40 mM or 10 mM) were similar, while the maximum hyperpolarization after application of 2.5 mM-calcium was smaller.4. The area of the maximal response evoked by adrenaline or ATP was independent of the exposure time to calcium-free solution after removal of the extracellular calcium (20 min). The sum of the areas of a first submaximal response, obtained by applying adrenaline for less than 5 min to the calcium-free solution (20 min), and of the second response (5 min application) elicited after continuing in calcium-free medium for another 8 min, was constant.5. In the presence of the bee toxin apamin (10(-7) M), addition of ATP (4 x 10(-4) M) caused depolarization of the membrane both in the presence and absence of external calcium. These responses were not blocked in low sodium solution (22.7 mM) but were reduced by the calcium antagonist D600 (2 x 10(-5) M).6. In calcium-free conditions the alpha-response to adrenaline was decreased by a preceding addition of ATP and vice versa. Abolition of the ATP response (4 x 10(-4) M) by adrenaline (10(-5) M) was prevented by blocking the alpha-receptors with phentolamine (2 x 10(-5) M).7. Carbachol (5 x 10(-7)-5 x 10(-5) M) depolarized the muscle cells in calcium-free medium; a second addition of carbachol also caused depolarization, the amplitude being lower. The carbachol depolarization was dependent on the exposure time to calcium-free solution.8. The adrenaline response was reduced by about 25% by carbachol if applied previously, independent of the carbachol concentration (5 x 10(-7)-5 x 10(-5) M). The carbachol response, however, was not affected if preceded by the alpha-response.9. It is concluded that ATP and the alpha-agonist, after binding to their receptor sites, activate the same mechanism, which is mobilization of calcium from the same membrane compartment to open potassium channels, causing hyperpolarization of the muscle cell membrane; the hyperpolarization is transient or sustained in nature depending on the availability of external calcium to replenish the calcium compartment localized in the membrane. This adrenaline and ATP-sensitive calcium compartment is distinct from that which is sensitive to carbachol.