The sensitivity of adult primate (Macaca mulatta) trigeminal ganglion neurons to capsaicin was studied using whole-cell recording techniques. Neurons responding to capsaicin (9 out of 14) generated inward currents of up to 3.0 nA (median = 0.23 nA; interquartile range = 1.19 nA) upon drug application measured at -60 mV. Capsaicin-sensitive neurons had longer action potential (AP) durations than capsaicin-insensitive neurons.