The possible potentiation by a rise in plasma calcium concentration of the effects of acetylcholine (ACh) on the atrial myocardium was investigated, mainly with a view to define the increase in vulnerability to fibrillation by hypercalcaemia. The effective refractory period (ERP) of the atrial myocardium, the atrial fibrillation threshold (AFT) and the atrial fibrillation rate (AFR) were measured repeatedly before and during the intravenous infusion of calcium at the rates of 0.025, 0.050 and 0.100 mmol . kg-1 . min-1 in dogs whose heart was, in addition, submitted to a cholinergic influence. 1. As long as the rise in plasma calcium concentration did not reach 100% approximately, this influence was enhanced considerably: in particular, ACh shortened ERP and raised AFR to a much larger extent, so that it resulted in fibrillation with a minor electrical stimulation. 2. When the rise in plasma calcium concentration exceeded 100%, hypercalcaemia became inhibitory of the effects of ACh, with a reversal in the modification of all the parameters, AFT especially, and, finally, prevention or even conversion to sinus rhythm of fibrillation.