A relatively simple method for obtaining the approximate temperature profile across a GaAs diode laser during a short, rectangular current pulse is described. The method, based on consideration of the thermal diffusion of an initially heated slab parallel to the junction, allows the inclusion of the thickness and distribution of more than one heat source. Results are valid for short times up to a maximum corresponding to a thermal diffusion length , where is the thermal diffusivity, equal to the junction depth. Numerical results are given for GaAs diodes operated at room temperature and some of the assumptions are verified experimentally. Finally, the results are used to estimate the shape and duration of the light output pulse in terms of threshold and drive current.