The susceptibility of sheep oocytes to temperature changes during maturation in vitro was tested by reducing the incubation temperature to 20 degrees C at various stages of meiosis. Cooling induced chromosomal abnormalities including disorganized metaphase plates and multipolar spindles in 28-54% of oocytes cooled at all stages of meiosis from germinal vesicle breakdown (GVBD) to metaphase II. The time of GVBD (8-11 h after the start of culture) was the most sensitive to cooling, whereas fewest abnormalities were found in oocytes cooled in late metaphase I (16-19 h). In addition to the chromosomal abnormalities, unusual vesicles appeared in the cytoplasm of oocytes cooled at 8-11 h and 12-15 h. No abnormalities in protein synthesis were detected by one-dimensional SDS gel electrophoresis. The consequences of the abnormalities for the developmental potential of the cooled oocytes were tested by transfer to recipient ewes and fertilization in vivo. After 12 days of development only 6% and 11% oocytes cooled at 12-15 h and 20-23 h respectively had developed to expanded blastocysts, compared with 44% of control oocytes. The results demonstrated that maturing sheep oocytes are very sensitive to a drop in temperature.