5'-N-Ethylcarboxamide adenosine (NECA) (2.2-22 nmol l-1) produced a concentration-dependent depression of both the isometric contraction and the atrial rate of the isolated, spontaneously beating guinea-pig atria. In the presence of a standard concentration of verapamil (73 nmol l-1), the dose-response curves for NECA, both for the isometric contraction and the atrial rate, were significantly shifted to the left. The inhibitory effect of NECA on the atria was almost completely reversed by the addition of calcium chloride (1.8 mmol l-1) into the organ bath. The depressive actions of NECA on the isolated atria were also antagonized by aminophylline (32 mumol l-1) and isoprenaline (24 nmol l-1). In the presence of XAC (Xanthine Amine Congener) (0.5 mumol l-1) the dose-response curve for the effect of NECA on the isometric contraction was significantly shifted to the right. In the concentration used, XAC produced no changes either in the isometric contraction or in the atrial rate. These experiments indicate that NECA may act on the adenosine A1-receptors present in the guinea-pig atria because its effects on the atria are antagonized by XAC, a more selective A1-receptor antagonist. It is concluded that NECA might act through depression of calcium channel function in the heart atria, and that the action of NECA most probably implicates A1-receptors.