The effect of acute and chronic nicotine treatment of rats on the mRNA levels coding for the three opioid peptide precursors, for provasopressin and for the c~3 subunit of nicotinic receptors in brain, pituitary and/or adrenal medulla of rats was investigated. Nicotine was found to increase the levels of proenkephalin mRNA in the adrenal medulla, but did not affect the levels of PENK mRNA in striatum, hypothalamus and hippocampus. The mRNA levels of prodynorphin were increased together with that of provasopressin in the hypothalamus after nicotine, whereas the prodynorphin mRNA levels in the hippocampus and the striatum remained unchanged. Nicotine treatment resulted in an increase in the pro-opiomelanocortin mRNA levels in the anterior pituitary and in a decrease in the intermediate pituitary, but did not change the levels of pro-opiomelanocortin mRNA in the hypothalamus. The levels of mRNA coding for the c~3 subunit of nicotinic receptors in the hypothalamus and the adrenal medulla remained unchanged. The increase in the prodynorphin and prova:~opressin mRNA levels in the hypothalamus was most pronounced ] day after s.c. application of two doses of 0.4 mg/kg nicotine (about 100% above control). A smaller increase in mRNA concentrations (about 30%) was found after tonic infusion of the drug for 4 days (4 mg/kg per day), whereas no change was observed after tonic infusion of nicotine for 7 and 14 days indicating the development of complete tolerance. The increase in proenkephalin mRNA levels in the adrenal medulla was highest after the short-term application of nicotine (about 150% above control). Less, but still significant increases in the mRNA levels