In the developing brain, histamine is one of the first neurotransmitters to appear. The concentration of histamine in the prenatal brain is fivefold that of adult levels. During the prenatal development a large transiently histamine-immunoreactive cell population distinct from the adult histaminergic system can be found within a subpopulation of the developing serotonergic raphe nuclei neurons. Also histamine-immunoreactive nerve fibers are widely distributed already during the prenatal development extending to the diencephalon, the thalamus, the cortex, and the spinal cord. Large numbers of histamine-containing mast cells also migrate into the brain during the late prenatal life. The wide distribution and high prenatal concentrations imply important functions for the histaminergic system during intrauterine development. However, little is known about the actual functions of histamine during development, and which of the histamine receptors are present in the prenatal rat brain is currently unknown. In the present study, we used in situ hybridization to study the distribution of H1-receptor (H1R) mRNA in the embryonic rat brain and spinal cord. H1R mRNA could be detected in rat brain and in spinal cord on embryonic day (E) 14, and the expression pattern seemed to partially localize in areas containing histamine-immunoreactive nerve fibers through E14-E20. H1R mRNA was also detected by reverse transcriptase polymerase chain reaction from embryonic brain samples and by Northern hybridization. The possible involvement of apoptosis in the disappearance of the developing transiently histaminergic system was studied by using apoptosis detection based on the terminal dUTP nick end labeling (TUNEL) technique and with c-Fos immunostaining. Although histamine immunoreactivity disappears dramatically from the developing raphe nuclei after E18, only occasional apoptotic nuclei could be seen in the histamine-immunoreactive cell bodies. The presence of H1R mRNA during the embryonic development renders it possible that histamine could exert an H1R-specific function at the time of the embryonic histamine peak.