A bridge circuit is described in this paper which permits tracing of both theI–Vand conductance curves of tunnel diodes. To insure accuracy, a constant voltage ac source employing feedback is used for the derivative measurement. Equipment used is relatively simple and the majority of it is commercially available. The circuit also allows fixed point measurements to be made at the user's option. Reproducibility and accuracy are mainly dependent on output indicator and the parameter being measured. Equations for small errors introduced by the bridge are presented (Appendix); they can be used to determine when a measurement is subject to excessive error. Measurements at other than room temperatures may be made with the bridge by physically isolating the high frequency circuitry containing the diode from the remainder of the bridge. Representative data from the bridge are presented.