Protons play a key role in nociception caused by inflammation and ischaemia, but little is known about the relative sensitivities of different dorsal root ganglion (DRG) neurons. We have therefore examined the responses in vitro of rat DRG cells classified according to whether or not they bind Griffonia simplicifolia IB4 (IB4), a lectin which is widely used to distinguish between two major populations of small diameter neurons. Under voltage-clamp conditions, proton-activated inward currents were found in approximately 90% of small DRG neurons and showed one of three waveforms: transient, sustained or mixed. The majority of IB4-positive (IB4+) neurons (63%) gave rise to sustained inward currents that were sensitive to capsazepine. In contrast, the most prevalent waveform in small IB4-negative (IB4-) neurons (69%) was a mixed response containing transient and sustained components. The transient component was inhibited by amiloride whilst the sustained component showed a variable sensitivity to capsazepine. We also found that more IB4+ cells responded to capsaicin and, on average, gave rise to a larger magnitude of response than small IB4- neurons, consistent with their higher prevalence and greater amplitude of vanilloid receptor 1 (TRPV1)-like acid responses. The increase in intracellular Ca(2+) induced by capsaicin was also slightly greater in IB4+ neurons and in these cells its magnitude correlated with the level of TRPV1 immunoreactivity. Our data suggest that acid-sensing ion channels (ASICs) and TRPV1 are the major acid-sensitive receptors in small IB4- neurons, whilst TRPV1 is the predominant one in IB4+ neurons. Because ASIC-like responses were approximately 10-fold more sensitive to changes in H(+) than TRPV1-like responses, we speculate that small IB4- rather than IB4+ neurons play an essential role in sensing acid. Our results also highlight differences in capsaicin responses between IB4+ and IB4- small neurons and reveal the close link between capsaicin responses and levels of TRPV1 expression.