In guinea pig taenia caeci smooth muscle the muscarinic receptor stimulant carbachol evoked depolarization and contraction, which was followed by hyperpolarization and relaxation on its removal. Both the hyperpolarization and relaxation were inhibited by removal of K+ from the external medium. During Na+-pump blockade (K+-free solution) the depolarizing and contracting actions of carbachol decreased. When the Na+ pump was switched on again by readmission of 5.9 mmol/L K+ to K+-depleted and Na+-enriched preparations, electrogenic hyperpolarization and relaxation developed. During this period carbachol failed to produce depolarization and contraction.