Some nociceptors produce and release Substance P (SP), a peptide cotransmitter with well-described effects on second-order sensory neurons. However, the effects of SP on primary sensory neurons are less clear. We tested the hypothesis that, among primary sensory neurons, SP targets mainly nociceptors. We therefore examined the biophysical and pharmacological profile of rat DRG sensorial neurons that are sensitive to SP. Whole-cell patch clamp was used to measure the response of 92-94 cells to brief applications of pH 7 (26 positive cells), pH 6 (38 positive cells), capsaicin (25 positive cells), and ATP (13 P2X3-type current positive cells). Sensitivity to SP was determined in 87 cells by its effect on cell excitability during stimulation at 1-4 times threshold. The number of action potentials at the threshold, and the slope of the stimulus-response curve were compared before and after SP application. We found that in 14 of the 87 cells examined, SP increased the slope of the stimulus-response curve, and in 12 cells SP increased the number of action potentials at the threshold. In total, 17 out of 87 cells tested (19%) increased excitability measured by one parameter or the other. There was also a decrease in excitability in 11 out of 87 cells. Among the cells responding to SP, the frequencies of sensitivity to pH 7 (61%) and pH 6 (78%) were statistically (Chi-squared test) higher than the frequencies of the non-responding cells (18% and 29% to pH 7 or 6, respectively). The frequencies of sensitivity to capsaicin had a tendency to be higher in the SP responding cells (20% versus 46%). P2X3-type ATP currents were present in 16% of cells that did not respond to SP, while one of the 17 (6%) substance P positive cells presented this current. Also, substance P responsive neurons had the largest diameters, and had broader action potentials with more pronounced after hyperpolarizations. We conclude that the majority of SP sensitive neurons exhibit a profile typical of nociceptors, although P2X3 currents were almost absent in these cells.