Rats were rendered dependent on morphine by repeated injections of morphine, in increasing doses for 14 days and sacrificed. Levels of peptides in the dorsal spinal cord and dorsal root ganglia were analyzed in rats decapitated 2 hr, 24 hr (acute abstinent) or 7 days (late abstinent) respectively, after the last injection of drug. Dynorphin A was significantly decreased in rats abstinent for 24 hr, while dynorphin B remained unaffected. Substance P and CGRP, both putative transmitters in nociceptive primary afferent neurones, and partly existing together in the same neurone, were affected differently. Significantly less substance P but unchanged levels of CGRP were detected in rats abstinent for 24 hr, while on the other hand, CGRP but not levels of substance P, were increased 2 hr after the final injection. In dorsal root ganglia, levels of substance P were lower at 2 hr, while levels of CGRP were unaffected. In late (7 days) abstinence, no effect of opiate on any peptide was detected.