The A1-adenosine receptor agonist, R-phenylisopropyl-adenosine (R-PIA), demonstrated antinociceptive properties in animal studies after intrathecal administration. In the evaluation of a drug for possible spinal injection in humans, the effects of intrathecal R-PIA on spinal cord blood flow (SCBF) were investigated using the laser-Doppler flow-metry technique in anesthetized rats. In low doses (0.1-1 nmol), no change in SCBF was recorded, whereas larger doses (10-100 nmol) caused a significant increase in SCBF. No change in systemic arterial blood pressure could be seen, except for a decrease after administration of the largest dose of R-PIA (100 nmol). It is concluded that R-PIA in doses of 10 nmol and larger induces an increase in SCBF after intrathecal injection in anesthetized rats and that an increase in blood flow is seen before any effect on the systemic circulation is detected. It can also be deduced that the antinociceptive effects of R-PIA after intrathecal injection are not a consequence of spinal ischemia and that disturbances in local blood flow cannot be expected to constitute a neurotoxic factor.