Growth factors and peptides playing important roles during early development of the central nervous system have also been shown to maintain their regulation of cell genesis in the adult brain. We have previously described that endogenous opioids, expressed in the developing hippocampus, regulate proliferation and differentiation in the adult rat hippocampus. The aim of this study was to investigate the effects of the opioid β-endorphin on gene expression and glial differentiation in cultures of adult rat hippocampal progenitors (AHPs). Changes in gene expression after stimulation of AHPs with β-endorphin for 48 h were investigated using cDNA arrays. Confirmation experiments verified that stimulation with β-endorphin increased the mRNA levels of myelin basic protein, glutathione S-transferase pi, c-junD and rab16 (P < 0.05), genes that are associated with oligodendrogenesis. Furthermore, β-endorphin increased the levels of Id1, but not Id3, mRNA on the arrays. Incubation of AHPs with β-endorphin resulted in a threefold increase in oligodendrogenesis (P < 0.01) but no significant change in astrogliogenesis. No effect on oligodendrogenesis was observed in the presence of the opioid antagonist naloxone. Coincubation of β-endorphin with Id1 antisense oligonucleotides for 10 days also entirely blocked the induced oligodendrogenesis in our AHP cultures. Moreover, a subpopulation of AHPs (25%) showed nuclear expression of the proneural transcriptional activator Mash1 that was reduced to approximately 5% of the cells when exposed to β-endorphin. We suggest a requirement for Id1 in opioid-induced oligodendrogenesis in cultured AHPs possibly acting on opioid-responsive AHPs expressing the proneural transcriptional activator Mash1.