Vicia faba root tips were exposed to concentrations of paraquat ranging from 10-3M to 10-6M. There was a statistically significant inhibition in 3H-thymidine incorporation, and, therefore, presumably of DNA synthesis by all concentrations of paraquat studied. All concentrations of paraquat also had a statistically significant effect on the percentage of cells in division at various hours following paraquat treatment. At 10-3M and 10-4M paraquat there was an almost total inhibition in the number of cells moving from the S period and through the G2 period and into division. Cells treated with 10-6M paraquat, however, did move through the G2 period and into division. A concentration of 10-4M paraquat did not result in an increase in chromosome aberrations.