Proton-proton spin-spin coupling in the system H-C-N-H has been observed in a series of N-methylanilines. At 25� in deuterochloroform those bases with pKa >3.5 show no coupling owing to the rapidity of proton exchange in solution. In pyridine, however, this upper pKa limit is raised to 4.6. The temperature-dependence of the coupling behaviour, and hence the proton exchange rates, have been qualitatively explored.