A method to predict the small-signal linear gain and level of harmonic distortion in analog MOS circuits is presented. This method, based on a generalized nonlinear transfer function approach, lends itself to implementation in the AC small-signal analysis routine of the circuit simulation program SPICE. A low-frequency nonlinear distortion model based on the CSIM simulator MOSFET model is applied to three simple MOSFET circuits. Results presented emphasize the need to consider small-signal quantities in the development of MOSFET models and in the determination of device parameters. The method can be easily extended to include capacitive effects and a prediction of intermodulation distortion.