Paraquat, a popular herbicide, is known to be very toxic to man and animals. Using cultured mammalian cells (human embryo lung cell, R-66), the effect of paraquat has been studied. With autoradiography, stimulation of DNA synthesis was observed when the cells were exposed to 0.08 or 0.4μM of paraquat; however, DNA synthesis was inhibited with 40 μM of paraquat. To determine cell growth rate, accumulation of mitotic cells was determined after addition of colcemid (0.2μg/ml) into the culture medium. The ratios of mitotic cells observed were proportional to the level of DNA synthesis. And the stimulation and inhibition of cell growth were observed even after washing off paraquat from media.