A new circuit model of a stripe-geometry double-heterojunction injection laser below threshold is presented. The model is derived from the physics of the semiconductor heterojunction, and takes into account the effects of active-layer carrier degeneracy, high-level injection, and nonradiative recombination along the stripe edge. Turn-on delay characteristics and small-signal input resistance characteristics of a laser are computed using the circuit model. Results of these analyses are compared with previous theoretical results and published experimental data.