A mode-conversion program has been developed which allows for the vertical inhomogeneity of the ionosphere as well as its anisotropy. As an illustration of the potential use of the program for diagnostic studies at VLF, the sunrise amplitude study of Walker  is numerically modelled with allowance for four modes. Exponential profiles of varying reference height and scale height are used to simulate the terminator. Of the models studied, best agreement is obtained with a terminator thickness of 1000 km. Despite the multimode nature of the models, Crombie's  model is found to be valid for the path studied. This is because of weak conversion from the third and fourth nighttime modes to the first daytime mode.