Four experiments were designed to examine the effects of colcemid, a microtubule assembly inhibitor, on the development of bovine nuclear transfer (NT) embryos in vitro and in vivo. Recipient oocytes matured at different times were exposed to colcemid. Approximately 80-93% of the exposed oocytes, with or without the first polar body (PB1), developed obvious membrane projections. In Experiment 1, oocytes matured for either 14-15 or 16-17 hr, treated with colcemid and used as recipient cytoplasm for NT resulted in over 40% blastocyst development. In Experiment 2, oocytes matured for 16-17 hr were treated with either 0.2 or 0.4 microg/ml colcemid for 2-3 or 5-6 hr, respectively. The percentages of blastocyst development (39-42%) were not statistically different among the different colcemid treatment groups, but were both higher (P < 0.05) than the control group (30%). Colcemid concentrations and length of colcemid treatment of oocytes did not affect their ability to support NT embryo development to the blastocyst and hatched blastocyst stages. Results from Experiment 3 indicate that semi-defined medium increases morula and blastocyst development of NT embryos derived from colcemid-treated oocytes under 5% CO2 in air atmosphere. In addition, cell numbers of blastocysts in colcemid-treated groups were numerically higher than the control groups. After embryo transfer, higher (P < 0.05) pregnant rates were obtained from the colcemid-treated group than the nontreated group. Five of 40 recipients (12.5%) which received embryos from colcemid-treated oocytes delivered healthy calves, significantly higher than those recipients (3.3%) that received embryos derived from nontreated oocytes.