The sensitivity of early preimplantation mouse embryos to drugs which disrupt microfilament function (cytochalasin B-CB and cytochalasin D-CD and microtubule assembly (colchicine, colcemid, vinblastine and griseofulvin) was examined. CD inhibited cleavage at a concentration 35-fold lower than CB (3 X 10-7 M v. 1 X 10-5 M). Treatment of 2-cell embryos for 6 h with 1 X 10-5 M CB or 1 X 10-6 M CD or continuous exposure to lower concentrations of CB or CD did not affect development to the blastocyst stage in vitro. Vinblastine inhibited cleavage at a concentration tenfold lower than colcemid or colchicine (1 X 10-8 M v. 1 X 10-7 M). The continuous presence of colcemid at 10-8 M did not affect the development of 2-cell embryos to the blastocyst stage, but development was reduced with vinblastine at 1 X 10-9 M and completely inhibited with colchicine at 1 X 10-8 M. The drugs produced similar responses when 2-cell embryos were treated for 6 h with concentration that inhibited cleavage. Complete inhibition of cleavage was obtained after only a 2 h exposure to 2 X 10-7 M colchicine. A similar concentration of lumicolchicine did not affect cleavage or blastocyst formation. Embryos were less sensitive to griseofulvin; the first cleavage division was unaffected by concentrations as high as 3 X 10-4 M and only 50% of 2-cell embryos failed to cleave in 1 X 10-4 and 3 X 10-4 M griseofulvin. At these concentrations a small proportion of 1-cell embryos and the majority of the 2-cell embryos showed unequal cytoplasmic division probably caused by the formation of multipolar spindles. The continuous exposure of 2-cell embryos to 3 X 10-5 M griseofulvin did not affect blastocyst formation.