The effect of transient high temperature in the uniaxial compressive strength of high strength concrete was investigated. The temperatures studied varied from 100\sp∘ to 800\sp∘C. The presence of loads in a structure was simulated by preloading the test specimens during the heating period. Results showed that exposure to temperatures between 100\sp∘ and 300\sp∘C decreased the compressive strength of high strength concrete by 15 to 20 percent. For temperatures between 400\sp∘ and 800\sp∘C,the compressive strength of concrete decreased to thirty percent of that at room temperature. One third of the preloaded specimens failed explosively during the heating period. In the remaining specimens the presence of a preload had a beneficial effect and a smaller loss of strength was observed compared to unstressed specimens. Exposure to high temperature caused the modulus of elasticity to decrease in all specimens regardlessly of the preload condition and the strength of concrete.