The antinociceptive effect of intracerebroventricularly (i.c.v.) administered [D-Arg1, D-Trp7,9, Leu11]-substance P (spantide), a non-selective tachykinin antagonist, was examined using the mouse formalin test. Licking behaviour induced by 2% formalin solution in the hindpaw of mice had two peaks, 0-5 min (first phase) and 10-30 min (second phase). I.c.v. spantide produced a dose-dependent antinociception during the first and second phases. The ID50 values were 2.95 (1.59-5.46) nmol for the first phase and 2.87 (1.49-5.52) nmol for the second phase. The antinociceptive effect in the first phase, but not in the second phase produced by spantide was antagonized by pretreatment with naloxone (1.0 mg/kg, i.p.), an opioid receptor antagonist. An opioid binding study using [3H]naloxone revealed that spantide was able to inhibit [3H]naloxone binding to mouse brain membrane preparations. These results suggest that opioid receptor systems in the mouse brain are involved in spantide-induced antinociception during the first phase, but not during the second phase of the formalin-induced nociceptive behaviour.