The medial prefrontal cortex (MPFC) has been described as a "visceromotor" cortical area, since autonomic effects such as depressor responses may be elicited from this area. The central circuitry which mediates these depressor responses may include a projection from the MPFC to the nucleus of the solitary tract (NTS). Neurones were recorded extracellularly in the MPFC and were tested for antidromic (AD) activation from the NTS. These were all tested for (1) constant spike latency, (2) ability to follow high-frequency stimulation to more than 200 Hz, and (3) where possible, collision of stimulation-evoked spike with spontaneous spike or spikes evoked by iontophoretic application of glutamate. Of the 34 cells studied, all had constant AD latency (30 +/- 1 ms, range 16-46 ms); they followed high-frequency stimulation up to 354 +/- 19 Hz, and only seven cells were spontaneously active (range 1-19 spikes/s). The threshold stimulation intensity for AD activation was 102 +/- 9 microA (n = 34, range 8-200 microA). Depth-threshold curves (n = 7) showed minimum-threshold AD activation currents that corresponded to the dorsal and ventral subdivisions of the NTS. Small shifts in AD latency were found in the depth-threshold curves, suggesting axonal branching. Analysis of recording sites showed that NTS-projecting MPFC neurones were predominantly found in the infralimbic and ventral prelimbic regions of the MPFC. These findings indicate that there is a population of neurones in the MPFC that projects to, and probably terminates within, the NTS. It is possible that this projection may, in part, mediate the cardiovascular response to MPFC stimulation.