Summary The response of lateral roots of Vicia faba to 8-ethoxycaffeine (8-EOC), theophylline, aminophylline, theobromine and dibutyryl cyclic AMP (db-cAMP) was studied. The methylxanthines were not equally effective in disrupting cells. 8-EOC was the most effective in inducing the formation of binucleate cells. These 8-EOC-induced binucleate cells did not divide. This inhibitory effect appears to work only on cells in mitosis during treatment. 8-EOC and colchicine show a mutual antagonism; each reverses, in part, the effects of the other drug. Nuclear volumes were determined in 8-EOC induced binucleate cells; their mean volume decreased from 383 to 323 μm3 in a 3 h recovery period following treatment. Exposure for 2 h to colchicine resulted in an increase in mean volume to 696 μm3; this is further evidence of the ability of colchicine to reverse some of the inhibitory effects of 8-EOC.