We have studied the effects of capsaicin, administered at concentrations found to be antinociceptive in behavioural tests, on nociceptive responses evoked both in spinal dorsal horn neurons in vivo and in spinal ventral roots in vitro. In halothane anesthetized rats, C-fibre evoked input produced by transcutaneous electrical stimulation in the peripheral receptive field was recorded from single wide dynamic range neurons located in superficial and deep dorsal horn of the lumbar spinal cord. This input was reduced by systemic administration of capsaicin at an antinociceptive dose (20 mumol/kg s.c.). Intradermal injections of capsaicin localized to the peripheral receptive field produced a transient increase in C-fibre evoked activity followed by a prolonged period of localized insensitivity to C-fibre stimulation. Spinal i.t. administered capsaicin also produced a rapid but reversible attenuation of peripherally evoked C-fibre input. In a neonatal rat spinal cord-tail preparation maintained in vitro, superfusion of the spinal cord with capsaicin (100-500 nM) produced a transient depolarization which was followed by an attenuation of responses to peripheral noxious heat and to spinal administration of substance P. Similar activity was produced by a prolonged superfusion of the spinal cord with substance P (50-200 nM). An HPLC method was used to estimate the concentration of capsaicin in a number of tissues following s.c. administration at an antinociceptive dose. In addition capsaicin concentrations were determined in the spinal cord following an i.t. administration.(ABSTRACT TRUNCATED AT 250 WORDS)