Results of a study of the electrical derivative characteristics of InGaAsP buried heterostructure lasers are presented. By considering current‐leakage paths and electrical contact nonlinearities, an equivalent circuit model appropriate to the buried heterostructure laser has been developed. Solutions for the theoretical electrical derivatives of series‐parallel‐reducible networks have been obtained and applied to the equivalent circuit model of the buried heterostructure laser. Comparison of experimentally measured and calculated electrical derivative characteristics provides both qualitative and quantitative information concerning the distribution of current and voltage in buried heterostructure lasers. By combining the techniques of electrical‐derivative measurement and equivalent‐circuit modelling, an improved understanding of the properties of buried heterostructure lasers has been established.