The human immunodeficiency virus (HIV) and influenza virus fusion peptides are approximately 20-residue sequences which catalyze the fusion of viral and host cell membranes. The orientations of these peptides in lipid bilayers have been probed with 15N solid-state nuclear magnetic resonance (NMR) spectroscopy of samples containing membranes oriented between stacked glass plates. Each of the peptides adopts at least two distinct conformations in membranes (predominantly helical or beta strand) and the conformational distribution is determined in part by the membrane headgroup and cholesterol composition. In the helical conformation, the 15N spectra suggest that the influenza peptide adopts an orientation approximately parallel to the membrane surface while the HIV peptide adopts an orientation closer to the membrane bilayer normal. For the beta strand conformation, there appears to be a broader peptide orientational distribution. Overall, the data suggest that the solid-state NMR experiments can test models which correlate peptide orientation with their fusogenic function.