Regulation of nitrate reductase mRNA by light and nitrate was studied in barley seedlings using a partial cDNA clone as a probe. Nitrate reductase mRNA was detected in roots and leaves within 40 min after supplying nitrate to the roots and reached peak accumulation at 2 h in the roots and at 12 h in the leaves. In the absence of nitrate, nitrate reductase mRNA was not detected in shoots but low levels were detected in roots. After reaching a peak, nitrate reductase mRNA declined to approximately 60% and 40% of the peak accumulation in the leaves and roots, respectively. In both roots and leaves the decline in nitrate reductase mRNA occurred while nitrate reductase activity and tissue nitrate were increasing. Light enhanced nitrate reductase mRNA accumulation but the responses of etiolated and green leaves to light were different. Seedlings grown in light and exposed to nitrate in the dark accumulated low but detectable nitrate reductase activity and mRNA. These seedlings did not respond to red, far-red, or blue light but did exhibit a strong fluence response to white light. However in etiolated seedlings, nitrate reductase mRNA increased 20-fold in response to red and blue light. These results indicate that phytochrome or another photoreceptor may facilitate induction of nitrate reductase transcription by nitrate in etiolated seedlings but not in green leaves.