Abstract: Mouse N18TG2 neuroblastoma and rat C6 glioma cell lines were injected into male nude mice, and the tumors were passaged serially. At each generation, tumors were analyzed for δ opioid binding using [3H][d-Ala2,d-Leu5]enkephalin and for σ1 and σ2 binding with 1,3-[3H]di-o-tolylguanidine in the presence and absence of 1 µM pentazocine. Receptor density (Bmax) and affinity (KD) were estimated by homologous competition binding assays. Opioid and σBmax values in the solid tumors were significantly lower than their original levels in vitro. KD values for opioid/σ ligands were similar in vitro and in vivo. With successive passages in the murine host, δ opioid and σ1 binding of the neuroblastoma-derived solid tumors became undetectable. In contrast, σ2 receptor Bmax values were unchanged with successive passages of the neuroblastoma-derived tumors and doubled in the nude mouse-borne gliomas. When neuroblastoma-derived solid tumors that were devoid of δ opioid binding were returned to culture, opioid receptors appeared to be up-regulated as compared with their original in vitro levels. Serial passaging of these recultured cells in vivo again resulted in a rapid decline in opioid receptor content. The opioid data are consistent with our prior findings on opioid binding diminution in human brain tumors. The pattern of change for σ binding was more complex, with the σ2 response in late passages of the glioma being reminiscent of the formerly observed increase in number of σ sites in transformed human meninges, kidney, and colon tissue.