Evidence for role of endogenous sex steroids in morphine antinociception.

The effect of morphine on responses to sustained mild pain was tested in male white rats with gonads and (or) adrenals removed. Morphine was ineffective in gonadectomized rats; adrenalectomy alone increased the effectiveness of morphine over that in sham-operated controls. Morphine antinociception may involve some actions of endogenous steroids.