The effects of paraoxon and atrazine on the spontaneously beating auricle, isolated from the heart of Sparus aurata, were assessed. Paraoxon, 5μM, eliminated the atria contraction within 28.4±2.8min, an effect which was fully reversed by 15μM atropine, an antagonist of muscarinic cholinergic receptors. The IC 50 was estimated to be 3.2±1.5μΜ. Atrazine, 50 and 100μM, induced a 22.5±3.2 and 32.9±2.3% increase in the force of auricle contraction, caused by excitation of sympathetic synaptic terminals releasing adrenaline. This effect was reversed by 50μM propranolol, a blocker of β-adrenoreceptors. The results have shown that both sympathetic and parasympathetic nerve terminals are activated by atrazine. Also, the auricle contraction is mainly under sympathetic control, while the frequency is dominated by cholinergic system. Finally, the detailed parameters of the auricle contraction estimated during exposure to specific pesticides, force, frequency, time-response curves and electromechanical coupling can be further used to assess and compare the toxic effects of other compounds, anticholinesterases for example, on the heart of the fish.