The degree of analgesia correlates to spinal morphine concentration after intrathecal administration in rats.

Antinociceptive effects of intrathecal injections of morphine and pethidine were evaluated in rats using the hot-plate and tail-flick tests. Both drugs elicited antinociceptive effects either it was evaluated with maximal response latency (sec.) or with area under the curve (AUC) (sec. X min.). The effects of pethidine were shortlived. The concentration of morphine in the spinal cord correlated to its antinociceptive effects when tested on hot plate. A minimal morphine concentration of 3 pmol/mg in the spinal cord was required to induce experimental analgesia in the rat.