The development of the enkephalinergic hypothalamoseptal tract in the guinea pig brain was studied from embryonic day 30 until birth. Proenkephalin (PE) mRNAs were detected in the hypothalamic magnocellular dorsal nucleus (MDN) by in situ hybridization with a synthetic 35S-labeled oligonucleotide. The Met-enkephalin-like immunoreactivity (Met-enk-LI) in the MDN and the lateral septum (LS) was detected with antibodies against Met-enkephalin, on adjacent cryostat sections. At the same time, an immunohistochemical study of the arrangement of enkephalinergic axon terminals in the LS at birth was performed at the electron microscopic level. PE mRNAs were first found to be expressed in the MDN at embryonic day 32 (E32) and increased to reach a maximal level at E48. Met-enk-LI was consistently detectable from E38 in numerous perikarya of the MDN as well as in nerve terminals of the LS. The number of Met-enk-LI cells of the MDN decreased after this stage until birth, whereas positive nerve endings in the LS increased. At the electron microscopic level, numerous cell bodies of the LS at birth were consistently surrounded by Met-enk immunoreactive nerve terminals. Cells expressing the PE gene and Met-enk-LI were also observed from E38 to E44 in the periventricular area. Some of these cells were found double-labeled with Met-enkephalin and Somatostatin antisera. The enkephalinergic system of the hypothalamoseptal tract appears at early embryonic stages and may be essential in regulating septal neuronal functions early in gestation. Differing ontogenic onsets of the enkephalinergic hypothalamoseptal and periventricular-median eminence tracts suggest possible developmental and functional differences.