Expression profiles of two recently isolated cDNA fragments, M1 and M2, expressed in the medial telencephalon, were analyzed in developing rat forebrain by in situ hybridization histochemistry and correlative immunocytochemistry. M1 expression was observed in the most ventral portion of the hippocampal rudiment with a sharp dorsal boundary, from embryonic day (E) 12 onward. Its location corresponded to the fimbrial anlage. As the fimbria developed, segregated expression of M1 and neuron-specific class III โค-tubulin or GAP-43 became apparent, suggesting that part of the neuroepithelium producing fimbrial neuroglia expresses M1. M2 expression in the E12 telencephalon was confined to part of the medial cerebral wall, including the presumptive preoptic region and hippocampus, with a diffuse dorsal boundary. At the same stage, M2 expression also occurred in part of the dorsal diencephalon adjacent to the M2-positive telencephalic region, with caudal extension along the dorsal midline, and in the zona limitans intrathalamica. M2 expression domains lacked neuron-specific class III โค-tubulin immunoreactivity. During later stages, M2 expression was found in association with the corpus callosum, hippocampal commissure, fimbria, optic nerve, stria medullaris, tract of the zona limitans, and habenulopeduncular tract. In most cases, M2 expression was detected in regions corresponding to fiber tracts prior to arrival of the earliest axons, which could be detected by TAG-1-or GAP-43 immunohistochemistry. These findings suggest that specialized cell populations express M1 and/or M2 genes in paramedian regions of the forebrain in advance of developing axonal pathways and may be involved in early specification of tract location and differentiation of tract neuroglia.