Cowpea seedlings were grown in earthen pots in normal sunlight and irrigated daily with 10 mM nitrate solution. These seedlings were supplied with a known concentration of 2,4-D, an auxinic herbicide, and harvested after 1, 3, and 7 days. The whole shoot tissue was used for various estimations. A decrease (12 to 28%) in total phosphorus, starch, dry weight, and nitrite reductase in treated plants was observed. However, a tremendous increase was observed in nitrate reductase activity (80%) in treated plants, both onin vivoandin vitroassays. 2,4-D-treated plants also had significantly higher endopeptidase activity. An increase in aminotransferases occurred on the 1st and 3rd days of treatment, and thereafter on the 7th day the activities were nearly the same as that in the parallel control. An increase in protein content of 18% on fresh weight and 46% on dry weight basis occurred. On the other hand changes in fresh weight were insignificant. NO3-content also remained nearly the same on the 1st and 3rd days but on the 7th day an increase of 19% in 2,4-D-treated plants was evident. The nitrate assimilation process in cowpea in light of these observations is discussed, and it is proposed that 2,4-D selectively affected the pathway of nitrate reduction.