1. Capsazepine is a synthetic analogue of the sensory neurone excitotoxin, capsaicin. The present study shows the capsazepine acts as a competitive antagonist of capsaicin. 2. Capsazepine (10 microM) reversibly reduced or abolished the current response to capsaicin (500 nM) of voltage-clamped dorsal root ganglion (DRG) neurones from rats. In contrast, the responses to 50 microM gamma-aminobutyric acid (GABA) and 5 microM adenosine 5'-triphosphate (ATP) were unaffected. 3. The effects of capsazepine were examined quantitatively with radioactive ion flux experiments. Capsazepine inhibited the capsaicin (500 nM)-induced 45Ca2+ uptake in cultures of rat DRG neurones with an IC50 of 420 +/- 46 nM (mean +/- s.e.mean, n = 6). The 45Ca2+ uptake evoked by resiniferatoxin (RTX), a potent capsaicin-like agonist was also inhibited. (Log concentration)-effect curves for RTX (0.3 nM-1 microM) were shifted in a competitive manner by capsazepine. The Schild plot of the data had a slope of 1.08 +/- 0.15 (s.e.) and gave an apparent Kd estimate for capsazepine of 220 nM (95% confidence limits, 57-400 nM). 4. Capsazepine also inhibited the capsaicin- and RTX-evoked efflux of 86Rb+ from cultured DRG neurones. The inhibition appeared to be competitive and Schild plots yielded apparent Kd estimates of 148 nM (95% confidence limits, 30-332 nM) with capsaicin as the agonist and 107 nM (95% confidence limits, 49-162 nM) with RTX as agonist. 5. A similar competitive inhibition by capsazepine was seen for capsaicin-induced [14C]-guanidinium efflux from segments of adult rat vagus nerves (apparent Kd = 690 nM; 95% confidence limits, 63 nM-1.45 microM). No significant difference was noted in the apparent Kd estimates for capsazepine in assays on cultured DRG neurones and vagus nerve as shown by the overlap in the 95% confidence limits.6. Capsazepine, at concentrations up to 1O microM, had no significant effects on the efflux of 86Rb+ from cultured DRG neurones evoked either by depolarization with high (50 mM) K' solutions or by acidification of the external medium to pH 5.0-5.6. Similarly capsazepine had no significant effect on he depolarization (50 mM KCl)-induced efflux of [14C]-guanidinium from vagus nerve preparations.7. Ruthenium Red was also tested for antagonism against capsaicin evoked ['4C]-guanidinium release from vague nerves and capsaicin induced 45Ca2" uptake in cultures of DRG neurones. In contrast to capsazepine the inhibition by Ruthenium Red (10-500nM in DRG and 0.5-10microM in vagus nerve experiments) was not consistent with a competitive antagonism, but rather suggested a more complex,non-competitive inhibition.