To clarify the role of brain temperature in permeability of the blood-brain barrier (BBB), rats were injected with methamphetamine (METH 9 mg/kg) at normal (23 degrees C) and warm (29 degrees C) environmental conditions and internal temperatures were monitored both centrally (nucleus accumbens, NAcc) and peripherally (skin and nonlocomotor muscle). Once NAcc temperatures peaked or reached 41.5 degrees C (a level suggesting possible lethality), animals were administered Evans blue dye (protein tracer that does not normally cross the BBB), rapidly anaesthetized, perfused and had their brains removed. All METH-treated animals showed brain and body hyperthermia associated with relative skin hypothermia, suggesting metabolic activation coupled with peripheral vasoconstriction. While METH-induced NAcc temperature elevation varied from 37.60 to 42.46 degrees C (or 1.2-5.1 degrees C above baseline), it was stronger at 29 degrees C (+4.13 degrees C) than 23 degrees C (+2.31 degrees C). Relative to control, METH-treated animals had significantly higher brain levels of water, Na(+), K(+) and Cl(-), suggesting brain edema, and intense immunostaining for albumin, indicating breakdown of the BBB. METH-treated animals also showed strong immunoreactivity for glial fibrillary acidic protein (GFAP), possibly suggesting acute abnormality or damage of astrocytes. METH-induced changes in brain water, albumin and GFAP correlated linearly with NAcc temperature (r = 0.93, 0.98 and 0.98, respectively), suggesting a key role of brain hyperthermia in BBB permeability, development of brain edema and subsequent functional and structural neural abnormalities. Therefore, along with a direct destructive action on neural cells and functions, brain hyperthermia, via breakdown of the BBB, may be crucial for both decompensation of brain functions and cell injury following acute METH intoxication, possibly contributing to neurodegeneration resulting from chronic drug use.