The effects of intrathecal administration of prostaglandins on pain responses in conscious mice were evaluated by using hot plate and acetic acid writhing tests. Prostaglandin D₂ (0.5–3 ng/mouse) had a hyperalgesic action on the response to a hot plate during a 3–60 min period after injection. Prostaglandin E₂ showed a hyperalgesic effect at doses of 1 pg to 10 ng/mouse, but the effect lasted shorter (3–30 min) than that of prostaglandin D₂. Similar results were obtained by acetic acid writhing tests. The hyperalgesic effect of prostaglandin D₂ was blocked by simultaneous injection of a substance P antagonist (⩾100ng) but not by AH6809, a prostanoid EP₁-receptor antagonist. Conversely, prostaglandin E₂-induced hyperalgesia was blocked by AH6809 (⩾500ng) but not by the substance P antagonist. Prostaglandin F_2α had little effect on pain responses. These results demonstrate that both prostaglandin D₂ and prostaglandin E₂ exert hyperalgesia in the spinal cord, but in different ways.