The assimilation of nitrate in plants involves the reduction of nitrate to ammonia in two steps. The first step requires nitrate reductase, a nitrate-inducible enzyme. When seedlings of squash ( Cucurbita maxima L.) were treated with nitrate, both nitrate reductase activity and protein were induced in the cotyledons. Poly(A) + RNA was prepared from cotyledons of nitrate-treated seedlings and was used to construct a λgt11 cDNA library. Using antibodies from mice immunized against purified nitrate reductase from squash, a recombinant λ phage was isolated that encoded part of the nitrate reductase mRNA. The antigens produced by the recombinant phage were used to affinity purify anti-nitrate reductase antibody from ascites fluid of immunized mice. The purified antibody bound to nitrate reductase protein on immunoblots and immunoprecipitated the enzyme from squash protein extracts. The cDNA insert (1.2 kilobases) hybridized to a 3.2-kilobase RNA that was 120-fold more abundant in nitrate-induced cotyledons compared with the uninduced tissue.