Experimental investigations of temperature rises produced in parts of the human body during exposure to microwaves of 10 and 9.4 cm wavelength are described. Application of linear heat flow theory shows that the experimental skin temperature rise in the initial stages of a microwave exposure is consistent with a thermal conductivity for tissues of 0.005 cal cm^-1 sec^-1 °C^-1. With occlusion of the blood supply to the irradiated region the conductivity remains at this value until the pain threshold is reached. With no occlusion, increase in blood supply due to heating causes the effective conductivity to rise during exposure, but the theory becomes inapplicable as soon as heat flow is non-linear. Comparison is made of theoretical and experimental subcutaneous and muscle temperatures after microwave exposure and the differences explained. The influence of wavelength variation and air-cooling of the skin are discussed.