<jats:title>Abstract</jats:title><jats:p>Brahman preimplantation embryos are less affected by exposure to heat shock than Holstein embryos. Two experiments were conducted to test whether the ability of Brahman embryos to resist the deleterious effects of heat shock was a result of the genetic and cellular contributions from the oocyte, spermatozoa, or a combination of both. In the first experiment, Brahman and Holstein oocytes were collected from slaughterhouse ovaries and fertilized with spermatozoa from an Angus bull. A different bull was used for each replicate to eliminate bull effects. On day 4 after fertilization, embryos ≥ 9 cells were collected and randomly assigned to control (38.5°C) or heat shock (41°C for 6 hr) treatments. The proportion of embryos developing to the blastocyst (BL) and advanced blastocyst (ABL; expanded and hatched) stages was recorded on day 8. Heat shock reduced the number of embryos produced from Holstein oocytes that developed to BL (<jats:italic>P</jats:italic> < 0.001, 55.6 ± 4.2% vs. 29.8 ± 4.2%) and ABL (<jats:italic>P</jats:italic> < 0.01, 37.7 ± 3.6% vs. 12.2 ± 3.6%) on day 8 as compared to controls. In contrast, heat shock did not reduce development of embryos produced from Brahman oocytes (BL = 42.1 ± 4.8% vs. 55.6 ± 4.8% for 38.5 and 41°C, respectively; ABL = 17.6 ± 4.2% vs. 32.4 ± 4.2%). In the second experiment, oocytes from Holstein cows were fertilized with semen from bulls of either Brahman or Angus breeds. Heat shock of embryos ≥ 9 cells reduced development to BL (<jats:italic>P</jats:italic> < 0.002) and ABL (<jats:italic>P</jats:italic> < 0.005) for embryos sired by both Brahman (BL = 54.3 ± 7.7% vs. 23.4 ± 7.7%; ABL = 43. ± 7.4% vs. 7.9 ± 7.4%, for 38.5 and 41°C, respectively) and Angus bulls (BL = 57.9 ±  7.7% vs. 31.0 ± 7.7%; ABL = 33.6 ± 7.4% vs. 18.4 ± 7.4%, for 38.5 and 41°C, respectively). There were no breed × temperature interactions. Results suggest that the oocyte plays a more significant role in the resistance of Brahman embryos to the deleterious effects of heat shock than the spermatozoa. Mol. Reprod. Dev. 63: 32–37, 2002. © 2002 Wiley‐Liss, Inc.</jats:p>