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Exposure of heifers to heat stress during oocyte maturation leads to embryos with reduced development. The objectives of the present study were threefold: 1) to evaluate effects of heat shock on oocyte function as assessed by cleavage and development, and associated effects on membrane integrity and protein synthesis, 2) to determine whether responses of oocytes to heat shock are modified by the presence of cumulus cells, and 3) to determine whether oocytes and 2-cell embryos are capable of synthesizing heat shock proteins in response to heat shock. Exposure of cumulus oocyte complexes (COCs) to 41 C did not alter the number of embryos that cleaved but reduced the number that developed to the blastocyst stage. In contrast, exposure to 420C reduced both cleavage and development rates. Maturation of COCs for 12 or 24 h at 410C or 420C did not affect membrane integrity as determined by fluorescein diacetate/ethidium bromide staining. However, exposure to 410C or 42 C reduced protein synthesis by oocytes when occurring during the first or last 12 h of maturation. Removal of cumulus cells prior to maturation reduced protein synthesis by oocytes; furthermore, exposure to 420C caused a greater percentage of reduction in protein synthesis in denuded oocytes than in those with intact cumulus. Analysis of [35S] methionine and [35S] cysteine-labeled proteins by two-dimensional SDS-PAGE and fluorography showed that nonmatured oocytes at 390C produced heat shock protein 68 (HSP68) and two other putative heat shock proteins of 71 and 70 kDa (P71 and P70, respectively); exposure to 42 C did not increase synthesis of any …