Heatstroke
Signs of heat stroke are severe, rapid panting, wide eyes, staggering and weakness.  If suspected and you can take  the animal's temperature rectally, any temperature above 106 degrees is dangerous.  The longer the temperature remains at or above 106 degrees the more serious the situation; heatstroke can be fatal in minutes!  Place your pet in a tub of cool running water or spray with a hose being sure the cool water contacts the skin and doesn't simply run off the coat. Thoroughly wet the belly and inside the legs. Take a rectal temperature if possible to know when to stop cooling.  A safe temperature is about 103 degrees.  Seek veterinary attention as soon as possible.

Pets left in cars are at a severe disadvantage when it comes to being able to dissipate heat from their bodies.  Even in the shade, and especially in humid conditions, dogs need to inhale air cooler than their normal body temperature of 102 degrees.  In fact, even 80 degree air temperatures can be dangerous.  Always be careful about leaving pets in cars during warm days... even a few minutes can be critical.