In the search for novel compounds to treat disorders of smooth muscle function, efforts have focused on some 2-substituted thieno[2,3-d]pyrimidin-4-one derivatives that show interesting spasmolytic action. Our laboratories have developed a new series of quaternary salts of 2-substituted thieno[2,3-d]pyrimidin-4-one and thieno[3,2-d]pyrimidin-4-one isomers with therapeutic potential. Thesesubstances were prepared starting from simple derivatives of thiophene. Their spasmolytic activity was evaluated on transmurally stimulated guinea-pig ileum. The most active compounds (IC50 1.12-2.71 microM) 7f-7h, 12d and 12f had the terminal piperidino nucleus in the thioalkyl chain and lacked two methyl groups in the thiophene ring. Their relaxant activity on the isolated ileum was potentiated (approx. 20-25%) by phosphodiesterase inhibitors. Compounds 7f-h, 12d and 12f were less effective in inhibiting contractions of the guinea-pig ileum induced by acetylcholine (IC50 26.7-41.4 microM) or histamine (IC50 41.5-63.4 microM) and had a moderate binding activity to muscarinic receptors in membrane homogenates from the rat heart (M2 sites; pKi values between 5.55+/-0.08 and 5.14+/-0.12; n = 3) and submaxillary gland (M3 sites; pKi values between 6.15+/-0.07 and 5.76+/-0.08; n = 3). Action involving soluble guanylyl cyclase or any potential binding to guinea-pig ventricular L-type calcium channels was not considered likely. It is concluded that at least two different mechanisms of action contribute to their spasmolytic activity.