According to the Oxford Textbook of Medicine vol. I section 6.92, death occurs when dehydration weight loss reaches 15-25 percent of your weight due to loss of tissue fluids and the increase in the concentration of salt in the remaining fluids.
Your core temperature is normally 38.6 C. When the ambient temperature exceeds this, conductive heat loss becomes inefficient and the core temperature rises. This results in hyperventilation due to panting, cerebral dysfunction resulting in confusion, and cessation of sweating and cardiovascular collapse. Above a core temperature of 41 C, cell death begins and by 51 C cells are dead within a few minutes.
Dehydration leading to heat stroke is the most serious threat at temperatures between 95 - 105 F without replenishing the lost water. In a recent medical symposium studying the effects of extreme temperatures on soldiers in operation 'Desert Storm', there were papers presented at the NATO/Aerospace Medical Panel Symposium held in Victoria, BC on 17-21 May 1993. With no cooling provides, a test of 19 volunteers determined that at 95 F, the initial symptoms of heat stroke ( headache, nausea) were present after about 290 minutes ( 5 hours), and within about 1- 1.5 hours at 105 F. There was a significant variation in the range of these onsets from individual to individual spanning nearly a factor of 2 each way.