Capsaicin is a pungent-tasting compound produced by plants in the Capsium family that activates a subset of primary afferent neurons associated with pain and thermoreception. Previous studies from dorsal root ganglion (DRGs) neurons suggest that many of capsaicin's physiological responses are a consequence of its activating a cation-selective current. To further characterize the responses to capsaicin whole-cell patch-clamp measurements were performed on rat DRGs to which 0.1-10 microM capsaicin was continuously applied. The capsaicin-activated currents exhibited marked variability in their thresholds, amplitude (to 15 nA), rates of desensitization, and the number of distinct maxima in the evoked current. Similar responses were found in rat trigeminal ganglion cells. The heterogeneity in the magnitude of the currents evoked by 0.1 microM capsaicin likely reflects different types of capsaicin-sensitive neurons; a result consistent with in vitro extracellular recordings from capsaicin-sensitive sensory afferents (Seno and Dray 1993).