The effect of elevated aqueous SO2 (0, 10, 20, 30, 40, 50, 100 and 250 ppm) on total RNA and DNA contents, Ribonuclease (RNase) activity, incorporation of radioactive isotopes of 3H-uridine and 14C-thymidine of pigeonpea (Cajanus cajan (L.) Millsp. cv. PDM1) and amaranth (Amaranthus paniculatus L. a local cultivar) leaf discs under light and dark conditions has been studied. The synthesis of RNA and DNA were reduced in both the leaf discs incubated in aqueous SO2. In between the pigeonpea and amaranth aqueous SO2 stimulated gradually with increase RNase activity was more in amaranth under dark than light. The 3H-uridine incorporation rate into RNA and 14C-thymidine incorporation rate into DNA was recorded higher values in amaranth than in pigeonpea observed under light conditions.