The 1H n.m.r. spectra of the following peptides have been investigated: (1) the encephalitogenic H-Phe-Ser-Trp-Gly-Ala-Glu-Gly-Gln-Lys-OH comprising residues 114-122 of bovine myelin basic protein, (2) the corresponding -Arg-OH peptide of human basic protein, (3) the inactive peptide in which D-Ala5 replaces L-Ala5. Measurements were made in D2O solutions at 270 and 600 MHz over a range of temperatures, concentrations and pH. All the proton resonances have been assigned by comparisons with other peptide data, titration shifts, selective decoupling and nuclear Overhauser effects, and data on the (α,α- 2H2)Gly7 nonapeptide. Ring current shifts and their temperature dependence indicated that there is preferential stacking of the Phe and Trp rings, and also interactions between these rings and the Gln and Lys residues near the C-terminus of the peptide. These data suggest a reverse turn at the Gly4-Ala5 residues, a conformation that would be consistent with results from energy calculations and biological activity. The n.m.r. spectra of the L-Ala and D-Ala peptides differed in the temperature coefficients of certain chemical shifts, including particularly those subject to ring current effects. Data in dimethyl sulfoxide were limited by effects of aggregation, but definite conformation differences compared to aqueous solutions were indicated.