We have isolated 4 cDNA clones (GRT1-4) encoding glutathione reductase (GR) from a tobacco (Nicotiana tabacum L.) leaf cDNA library. The cDNAs were almost identical: GRT1, GRT3 and GRT4 represented the same gene, differing only in that GRT4 contained an intron within the C-terminal part of the coding sequence. Failure to splice out this intron resulted in a substitution of the final 13 amino acids of the deduced amino acid sequence. A second gene was represented by GRT2. Southern blots indicated that there were two related GR genes in tobacco. The presence of multiple isoforms of GR in tobacco may be explained in part by the expression of a small gene family. In addition, alternative isoforms may result from translation of different mRNAs derived from the same gene by intron skipping during the splicing of nascent GR mRNAs.