In vitro Heat stress (41 ?C) may have adverse effects on oocyte maturation, early embryonic development and cellular functions. The precise physiological causes that result in reduced development of oocytes after heat shock (HS) are not clear. The present study was conducted to investigate the effects of heat shock (HS) during in vitro maturation of ovine oocyte on the nuclear maturation (development to the metaphase II stage), early embryonic development, total cell number per blastocyst and mRNA content. In addition the effect of heat shock was studied on zona pellucid hardening, oocyte diameter and patern of cortical granule distribution. Cumulus–oocyte complexes were cultured under standard IVM conditions for 22-24 h at 38.5 and 41.0 ?C for the ?rst 12 h of maturation. Incubation during the last 10 h of maturation was at 38.5?C and in 5% (v/v) CO2 for both treatments. Immunostaining studies revealed that oocytes matured at 38.5 ?C were mostly at metaphase II (MII) stage, while the most populations of heat-shocked oocytes were blocked at the germinal vesicle break down (GVBD) stages and had an aberrant chromosome. While rates of blastocyst formation and hatched blastocyst didn’t differ significantly between two groups, cleavage rates (89.28 vs. 56.19%, p 0.05), and percentage of morula (26.85 vs. 37.81%, p 0.05) were significantly decreased in 41 vs. 38.5 ?C. The quality of embryo (ICM cell number) significantly (p= 0.007) decreased in heat shock compared to control group. Zona pellucida hardening (solubility to 0.5% pronase), oocyte diameter and cortical granule distribution (lens culinaris agglutinin ?uorescein isothiocyanate [FITC- LCA]) were evaluated in matured oocytes. Heat-shock increased the diameter of oocyte (p 0.05) and Zona pellucida hardening (p 0.05) followed in vitro maturation. Results indicate that heat shock disrupts nuclear maturation and had a venomous effect on developmental competence. Immuno?uorescence microscopy revealed impaired migration of cortical granules in oocytes matured at 41 ?C compared with oocytes matured at 38.5 ?C. Percentages of oocytes with incomplete migration of cortical granules were signi?cantly higher in oocytes matured at 41.0 ?C than in oocytes matured at 38.5 ?C Quantitative expression of six genes [Heat Shock Protein 90 (HSP90), OCT4, Poly A Polimerase (PAP), Cyclin b, Na/K-ATPase and connexin 43(CX43)] by Real- Time PCR showed a signi?cantly (p 0.05) higher transcripts abundance of Na/K-ATPase mRNA in heat shocked oocytes. Results were normalized against the GAPDH housekeeping gene. These alterations are likely to be involved in the mechanism underlying heat shock-induced disruption of oocyte capacity for fertilization and subsequent development. In summary, heat stress can have disruptive effect on oocyte during in vitro maturation and impairs its cellular and molecular events that important for subsequent development. Key worlds: heat shock, oocyte, nuclear maturation, mRNA content