There is substantial evidence that the intra- and intercellular messenger nitric oxide, generated enzymatically from L-arginine by nitric oxide synthase in different isoforms, is involved in the development of nervous tissue. In this study we investigated the nitric oxide expression in the pre- and postnatally developing rat brain. With regard to messenger RNA, all of the basic nitric oxide synthase isoforms (neuronal, endothelial and macrophage nitric oxide synthase) were already expressed at embryonic day 10 and showed a temporary decrease at embryonic day 17. Western blot analysis of the three isoform proteins revealed a time pattern that was different from those of messenger RNAs. Although the endothelial nitric oxide synthase isoform was also expressed at embryonic day 10, no quantitative changes were observed over the whole time period studied. Protein amounts of brain and inducible nitric oxide synthase were first detectable at embryonic day 15, with a tendency to rise. A parallel time pattern was found for the NADPH-diaphorase activity in our light microscopic studies, whereas ultrastructurally the reaction product was seen in the brain pallium even of 13-day-old embryos. The data indicate a permanent presence of the transcripts for all nitric oxide synthase isoforms in the rat central nervous system from embryonic day 10 onwards, although the expression of respective proteins and staining patterns may differ.