ATP-activated P2X₂ and P2X_2/3 receptors mediate slow inward currents in a subset of rat dorsal root sensory neurons (Liu et al. 2000) and, in those cells, are inhibited by vanilloid (TRPV1) receptors activated tonically by capsaicin (0.5 μM) (Piper & Docherty, 2000). Here, we identify another inhibitory action of TRPV1 receptors, when activated transiently by capsaicin (0.3-10 μM), on ATP-gated P2X₃ receptors which mediate fast inward currents in sensory neurons. Rat isoforms of TRPV1 and P2X₃ receptors were expressed alone, or together, in defolliculated Xenopus oocytes and studied under voltage clamp conditions (Axoclamp 2B amplifier; headstage configuration: ME1, x1LU; ME2, x10MGU; input range, ±13 μA). To avoid saturating the current-following headstage, receptor expression levels were kept low by injecting small volumes (20-40 nl) of complementary RNA (1 mg ml^-1) into oocytes, and the amplitude of evoked currents also kept low (1-2 μA) by holding cells at -30 mV. With P2X₃ receptors alone, rapidly inactivating and pH-insensitive inward currents were evoked by ATP (EC₅₀, 1.3±0.3 μM; mean ± SEM, n = 6). ATP potency was unaltered (1.2±0.4 μM; n = 6) by the presence of capsaicin (10 μM) which was not an agonist. With TRPV1 receptors alone, slowly inactivating and pH-sensitive inward currents were evoked by capsaicin (EC₅₀, 0.86±0.07 μM; n = 6) and its potency was unaffected (0.88±0.05 μM; n = 6) by the presence of ATP (10 μM) which was not an agonist. With TRPV1 and P2X₃ receptor coexpression, the simultaneous activation of both sets of ion channels produced independent responses seemingly unaffected by the activation of the other ion channel population. However, the prior activation of TRPV1 receptors led to a time- and amplitude-dependent inhibition of ATP responses at P2X₃ receptors. Inhibition of ATP responses was characterised by a reduction (69±2.3%, n = 5) in agonist efficacy without altering its potency. Inhibition was dependent of extracellular pH, since acidification potentiated capsaicin responses. Similar to the case of P2X₂/P2X_2/3 receptor inhibition (Piper & Docherty, 2000), the inhibitory action of TRPV1 receptors on P2X₃ receptors was also dependent on extracellular Ca²⁺ ions. Inhibition was independent of the holding potential in voltage clamp experiments. The prior activation of P2X₃ receptors did not affect the amplitude of capsaicin responses. The observed inhibition of P2X₃ receptors by TRPV1 receptors joins a growing list of examples where P2X purinoceptors are affected by non-purinergic ion channels, and occasionally vice versa. This particular example draws attention to a therapeutic regulatory interdependence between these ion channels in sensory neurons, since TRPV1 receptor activation by either heat or another stimulus may down-regulate purinergic signalling via P2X₃ (and P2X_2/3) receptors in nociceptive sensory neurons.