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The hypothalamic-pituitary-adrenal axis (HPA axis) is activated by various stresses. The objective of this study was to observe the acute heat exposure on the expression of hypothalamic corticotrophin-releasing hormone (CRH) mRNA and content of serum adrenocorticotropic hormone (ACTH) and corticosterone (CORT) in rat, and investigate hypothalamic neurons proliferation and differentiate. Male Wistar rats (160-180g) were randomly divided into two groups and exposed to heat at 32 degree (heat exposure, HE) or to a room temperature at 24 degree (control, CN). After 7 days of heat exposure, the body core temperature was measured by telemetry (n=4). The relative weight of the pituitary and adrenal gland were determined (n=4), and the expression of hypothalamic CRH mRNA was detected by using real-time fluorescence quantitative polymerase chain reaction (RT-PCR), and content of serum ACTH and CORT were detected by applying enzyme-linked immunosorbent assay (ELISA) (n=6). Another batch rats (male, 180-200g, n=4) was intraperitoneally injected Bromodeoxyuridine (BrdU) daily (50mg/kg) for 6 consecutive days. After commencing heat exposure, on the 7th day of exposure to heat, rats' brains were removed. Immunohistochemical analysis the numbers of BrdU-positive cells and double-stained by a mature neuron marked cell in the hypothalamus were observed. All rats were anesthetized via intraperitoneal administration of petobarbital sodium (70 mg/kg). Values are means ± S.D., compared by ANOVA. Compared with control group, heat exposure did not obviously affect body core temperature (p>0.05) or body weight of the rats (p>0.05). Seven days heat exposure increased the relative weight of pituitary and adrenal gland (e.g. CN: 0.026±0.002 vs. 0.17±0.02 mg/g, HE: 0.031 ±0.001 vs. 0.24±0.02 mg/g, p<0.05 respectively), and significantly increased ACTH and CORT contents of serum (e.g. CN: 11.66±1.21 vs. 12.38±3.21 mmol/L, HE: 15.23±2.09 vs. 18.45±2.06 mmol/L, p<0.05, p<0.01 respectively). The CRH mRNA expression levels increased but had no statistical difference (p>0.05). The number of BrdU-positive cells were significantly increased (67.8±11.2 for control rats and 117±12.3 for heat exposure, p<0.05), while the number of BrdU-positive cells double-stained by a mature neuron marker no significantly increased after 7 days of heat exposure. Acute heat stress affect hypothalamic pituitary-adrenal (HPA) axis, and may lead to the activation hypothalamic proliferation. [ABSTRACT FROM AUTHOR]