Rabbits chronically implanted with permanent cannulae were used in brain perfusion and microinjection experiments. Potassium iontophoresis applied to the rabbits' ear skin served as a noxious stimulus and the electric current used to elicit the defense response was taken as the pain threshold. The brain perfusate was analysed by radioreceptor assay and the level of endogenous opioid peptides (EOP) was expressed as competition rate. Electroacupuncture (EA) elicited an increase in pain threshold and a rise in EOP level in the perfusate from the anterior part of the head of the caudate nucleus (n = 10, P less than 0.002) but not from the posterior part. The pain threshold raising effect of EA could readily be reversed by microinjection of naloxone into the anterodorsal part of the head of the caudate (n = 12, P less than 0.01). With the techniques of multimicropipettes and microiontophoresis, caudate neuronal activity was recorded and examined in acute animals anesthetized with chloralose and urethane. It was found that microiontophoresed etorphine produced a strong, naloxone reversible inhibition of the spontaneous activity of the caudate neurons (61/162). Most etorphine sensitive neurons were identified in the dorsal part of the head of the caudate (P less than 0.01). EA produced inhibition of some etorphine sensitive neurons (16/35) and the inhibition could also be reversed by microiontophoresis of naloxone (4/8). The results indicate the participation of intracaudate opioid peptides in acupuncture analgesia.