Background information. Many neurons secrete classical transmitters from synaptic vesicles as well as peptide transmitters from LDCVs (large dense-core vesicles). Little is known about the mechanistic differences between these two secretory pathways. The soluble protein Munc18-1 is essential for synaptic vesicle secretion [Verhage, Maia, Plomp, Brussaard, Heeroma, Vermeer, Toonen, Hammer, van den Berg, Missler, et al. (2000) Science 287, 864-869]. Results. In the present study, we tested if Munc18 genes are also involved in peptidergic secretion from LDCVs using the anterior pituitary as a model system. We show that Munc18-1 is the dominant isoform expressed in the anterior pituitary. In Munc18-1 null mutant mice, the anterior pituitary developed normally and the five major endocrine cell types had a normal distribution. However, circulating peptide hormone levels were decreased by up to 50-fold in the null mutant, whereas the intracellular levels were significantly higher than that in controls. Ultrastructural analysis using the tannic acid method revealed striking differences in the distribution of secretory vesicles: (i) the number of exocytotic figures was mostly decreased in the null mutants and (ii) the LDCVs accumulated near but not at their target membrane. This is in contrast with the apparently normal distribution of synaptic vesicles in developing synapses in the null mutant (Verhage et al., 2000). Conclusions. We conclude that Munc18-1 is involved in the secretion of peptide hormones and in the docking of LDCVs. These results unmask an apparent mechanistic difference between LDCVs and synaptic vesicles.