Eighteen 2.2-ma and four 10-ma P-substrate (N-on-P), and eighteen 2.2-ma and four 10-ma N-substrate (P-on-N) substrate tunnel diodes were irradiated to 1.5 ?? 1016 n/cm2 (E > 0.3 Mev) and 2.2 1010 ergs/gm-(C) to investigate their radiation-resistance. Peak currents were found to remain constant under these exposures while valley currents increased due to the increase in excess current. P-substrate diodes were found to be the more radiation-resistant, degrading about one half as much as the N-substrate units.