ELF- and transient magnetic fields at moderate flux densities generate visual phenomena called magnetosphosphefles. The aims of this study were 1) to determine threshold values for the appearance of magnetophosphenes as a function of magnetic-field frequency in normal and colour-defective volunteers; 2) to study the dependence of the threshold values on the intensity of the background light and its spectral composition; and 3) to investigate the site in the visual system where the magnetophosphenes arise. Quantitative information was obtained about the threshold values of magnetophosphenes both in normal and in colour-defective volunteers. The phosphene threshold varied with the background light intensity and with its spectral composition.