Co-cultures of rat ventricular myocytes and sympathetic neurons were established. Superior cervical ganglia and ventricles from newborn rats were enzymatically dissociated and plated in a culture dish. Experiments were done between the 3rd (when evidence of neuron-myocyte proximity arises) and the 5th day in culture (before the myocytes become confluent). Simultaneous intracellular recording from a cardiomyocyte and an attached neuron was done using conventional microelectrode techniques (resistance of 60-100 Mohm). The myocytes in co-culture were either quiescent or spontaneously contracting. The contracting cells were either latent pacemaker or ventricular-like myocytes. The action potential (AP) characteristics of cardiomyocytes in co-cultures were comparable to those recorded in cardiomyocytes in pure cultures. Sympathetic innervation of the cardiomyocytes in co-cultures was evidenced by stimulating the neuron and observing an increase in rate of beating in latent pacemaker myocytes (average increase of 19.4 +/- 4.6%). In quiescent cardiomyocytes, neural stimulation evoked a slow depolarization that can reach threshold and initiate APs in the cell. This response is similar to slow excitatory postsynaptic potentials (EPSPs) observed in other synapses. Slow ESPSs could also be recorded in spontaneous beating cells, made quiescent by nifedipine (1x10(-6)-1x10(-7) M). These results indicate that functional synaptic contacts are developed in co-culture of sympathetic neurons and cardiac myocytes, and slow EPSPs can be evoked in cardiomyocytes as well as in other excitable cells. The sympathetic innervation occurring in culture did not significantly modify the spontaneous AP characteristics of the cardiomyocytes.