1H-NMR and 15N-NMR signal assignments have been made for the eight arginine residues in Lactobacillus casei dihydrofolate reductase in its binary complex with methotrexate and in its ternary complex with methotrexate and NADPH. 1H-NMR chemical shifts for the guanidino groups of two of the arginines (Arg57 and Arg43) were sensitive to different modes of binding of the guanidino groups with charged oxygen atoms of the ligands. In the complexes formed with methotrexate, Arg57 showed four non-equivalent NH eta proton signals indicating hindered rotation about the N epsilon-C zeta and C zeta-N eta bonds. The NH eta 12 and NH eta 22 protons showed large downfield shifts, which would be expected for a symmetric end-on interaction of these protons with the charged oxygen atoms of a carboxylate group in methotrexate. These effects were not observed for the complex formed with trimethoprim, which does not contain any carboxylate groups. In the complex formed with NADPH present, Arg43 showed a large downfield chemical shift for its NH epsilon proton and a retardation of its rate of exchange with water. This pattern of deshielding contrasts with that detected for Arg57 and is that expected for a side-on interaction of the guanidino group protons with charged oxygen atoms of the ribose 2'-phosphate group of NADPH.