Seasonal nitrate reductase (NR) activity of young, uppermost leaves of Atriplex canescens (Pursh)Nutt. (diploid genotype) growing in situ was maximal during reproductive growth. Because precipitation on the study site coincided with reproductive growth, higher NR activity during this periods may have been due to new leaf growth and increased soil NO³ availability rather that the nitrogen sink in developing seeds. Seasonal leaf water content (percent) was significantly ( P < 0.10 ) correlated with NR activity. Stem xylem water potentials were not correlated with NR activity. Although thermal adaptation of NR would, in general, be advantageous for plants inhabiting a desert environment with widely fluctuation seasonal temperatures, there was no evidence to suggest that acclimation of NR occurs in A. canescens growing in situ.