In the present experiments, the time course of the analgesic response (tail flick in rats, skin twitch in cat) and blood pressure effects of intrathecally administered morphine and the alpha-agonists, clonidine and ST-91 were examined in rats and cats. The clonidine analogue, ST-91 has approximately 1/50 the lipid solubility of clonidine. Intrathecally administered morphine and morphine followed by naloxone had no effect on resting blood pressure in unanaesthetized cats and in rats and cats anaesthetized with chloralose-urethane. Inactive when given systemically, ST-91 had a powerful antinociceptive effect when given intrathecally. At analgesic doses, this alpha-agonist produced only a transient hypertension. At doses 4-6 times that required to produce a significant antinociceptive effect, a pressor effect was observed. Intrathecal injection of clonidine in smaller doses produced a transient hypotension, with significant pressor effects present with larger doses.