In adult mammalian cardiomyocytes, stimulation of muscarinic receptors counterbalances the β‐adrenoceptor‐mediated increase in myocardial contractility and heart rate by decreasing the L‐type Ca²⁺ current (I_Ca) (1, 2). This effect is mediated via inhibition of adenylyl cyclase and subsequent reduction of cAMP‐dependent phosphorylation of voltage‐dependent L‐type Ca²⁺ channels (3). Little is known, however, about the nature and origin of this pivotal inhibitory pathway. Using embryonic stem cells as an in vitro model of cardiomyogenesis, we found that muscarinic agonists depress I_Ca by 58 ±3% (n=34) in early stage cardiomyocytes lacking functional β ‐adrenoceptors. The cholinergic inhibition is mediated by the nitric oxide (NO)/cGMP system since it was abolished by application of NOS inhibitors (L‐NMA, L‐NAME), an inhibitor of the soluble guanylyl cyclase (ODQ), and a selective phosphodiesterase type II antagonist (EHNA). The NO/cGMP‐mediated I_Ca depression was dependent on a reduction of cAMP/protein kinase A (PKA) levels since application of the catalytic subunit of PKA or of the PKA inhibitor PK) prevented the carbachol effect. In late development stage cells, as reported for ventricular cardiomyocytes (2, 4), muscarinic agonists had no effect on basal I_Ca but antagonized β‐adrenoceptor‐stimulated I_Ca by 43 ±4% (n=16). This switch in signaling pathways during development is associated with distinct changes in expression of the two NO‐producing isoenzymes, eNOS and iNOS, respectively. These findings indicate a fundamental role for NO as a signaling molecule during early embryonic development and demonstrate a switch in the signaling cascades governing I_Ca regulation.—Ji, G. J., Fleischmann, B. K., Bloch, W., Feelisch, M., Andressen, C., Addicks, K., Hescheler, J. Regulation of the L‐type Ca²⁺ channel during cardiomyogenesis: switch from NO to adenylyl cyclase‐mediated inhibition. FASEB J. 13, 313–324 (1999)