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Global climate change is predicted to intensify thermal stress in marine and coastal organisms, affecting their development, growth, and reproductive functions. In this study, we performed histological observations on ovarian development, immunohistochemical analyses of ovarian heat shock protein-70 (HSP70), nitrotyrosine protein (NTP, an indicator of reactive nitrogen species (RNS)), and dinitrophenyl protein (DNP, an indicator of protein oxidation) expressions, in situ TUNEL assay for cellular apoptosis, biochemical analyses of ovarian caspase-3/7 activity and protein carbonyl (PC, a measure of reactive oxygen species (ROS)) contents, nitrate/nitrite (NOx) levels, and extrapallial fluid (EPF, an important body fluid) pH in the American oyster, Crassostrea virginica. Oysters were exposed to medium (28 degrees C) and high (32 degrees C) temperatures under controlled laboratory conditions for 1 week. Oysters exposed to higher temperatures significantly decreased the number and diameter of eggs, and EPF protein concentrations compared with controls (24 degrees C). In contrast, EPF pH, ovarian HSP70 mRNA levels, and protein expression were increased after heat exposure, consistent with increased ovarian apoptosis. The enhanced apoptosis in ovaries was associated with increased ovarian caspase-3/7 activity, PC contents, NOx levels, and NTP and DNP expressions in heat-exposed oysters. Collectively, these results suggest that higher temperatures drastically increase RNS and ROS levels, increasing incidence of apoptosis and subsequently reducing ovarian functions in oysters.