Systemic administration of the 5-HT1A receptor agonist 8-OH-2-(di-n-propylamino)-tetralin (8-OH-DPAT; 0.3 mg/kg, s.c.) was used to explore the effects of activation of 5-HT1A receptors on expression of mRNA coding for 5-HT1A receptor, tryptophan hydroxylase (TPH) and galanin in the ascending raphe nuclei. 8-OH-DPAT increased the hybridization signal of the 5-HT1A receptor by 105% in the dorsal raphe nucleus (B7) 30 min after the injection. No effects were seen at the later time points (2-8 h). In the median raphe nucleus (B8) and the B9 cell group in the medial lemniscus, 8-OH-DPAT induced a marked decrease in labeling 30 min after injection. At 8 h following 8-OH-DPAT injection, the effect had shifted to an increase in 5-HT1A receptor labeling by 68% in the B8 area. Importantly 8-OH-DPAT had no significant effects on the expression of mRNA coding for TPH and galanin. The results suggest an important and differential mechanism for the regulation of 5-HT1A receptor mRNA levels in the dorsal and median raphe nuclei. This regulation may be of importance for the differential control of the activity of the ascending 5-HT neurons, and hence for mood regulation. The results also indicate a dissociation between the effects mediated by 5-HT1A receptor functions and those regulating the coexisting peptide galanin in the dorsal raphe.