Ninety-six h post insemination (hpi), embryos with 16 cells were divided in two groups: control and HS. In the control group, embryos were cultured at 39oC, whereas in the HS group, embryos were subjected to 41oC for 12 h, and then returned to 39oC. Rates of cleavage, and formation of morula and blastocysts were higher(P 0.05) for oocytes aspirated at 4.0 versus 6.5 h after ovaries were acquired.