The aims of this study were to determine whether a relationship exists between the occurrence of coronary artery occlusion-induced arrhythmias in the anaesthetized rat and the levels of cyclic AMP and cyclic GMP in both normal and ischaemic myocardium, and to assess whether such arrhythmias were modified by pretreatment with the phosphodiesterase inhibitors, quazodine and isobutyl methylxanthine (IBMX), or with the butyryl derivatives of cyclic AMP and cyclic GMP. At 5 min after coronary artery ligation (when only a few arrhythmias had occurred) both cyclic AMP and cyclic GMP levels were elevated in normal myocardium whereas in ischaemic tissue only cyclic AMP was raised. As the peak of the arrhythmic activity and after cessation of the arrhythmias, i.e. at 10 and 30 min post-ligation respectively, levels of both nucleotides had fallen in ischaemic although not in normal tissue. The severity of these occlusion-induced arrhythmias was exacerbated by pretreatment intravenously with quazodine, IBMX, dibutyryl cyclic AMP and dibutyryl cyclic GMP. Pretreatment with IBMX was also shown to elevate significantly both cyclic AMP and cyclic GMP content of left ventricular tissue before occlusion. None of the drug pretreatments markedly affected mean arterial blood pressure but heart rate was significantly increased following quazodine and IBMX administration. We conclude that in the pentobarbitone-anaesthetized rat the occurrence of occlusion-induced arrhythmias was not accompanied by a rise in cyclic nucleotide content of the ischaemic myocardium but agents which may elevate either myocardial cyclic AMP or cyclic GMP levels exacerbate such arrhythmias.