In an effort to increase gigantism in diploid snapdragon 2n=16 and larkspur plants 2n= 16, the present study was undertaken to induce polyploidy by colchinization. Colchicine was applied as soaking seeds or to the emerging apical buds of transplants at 0.05, 0.1, 0.2 and 0.4 percent aqueous concentration employing cotton plugs for 24, 48, 72 and 96 hrs. The investigated parameters were emergence rate, stem length, stem thickness, leaf characteristics and flower characteristics. Results revealed that soaking seeds in 1% colchicine and 24-hour operating time induced the highest numbers of tetraploid plants in Antirrhinum majus, and 0.5 % for 48 hour gave the same result in Delphinium ajacis. Expression of gigantism in morphological characters such as plant height, size of leaves and flowers were observed in tetraploid plants. Studies on morphological characters showed that the colchicine exhibited delayed reduction in number of leaves per plant. There was reduction in the number of flowers per plant and floral variants observed including bigger sized flowers.