Auditory brainstem-evoked electrical responses of guinea pigs were used to determine microwave energy thresholds for perception of pulsed radiations. A klystron was used to generate 918-MHz microwaves at pulse widths between 10 and 500 μs; the microwaves were fed into a circular waveguide that partly enclosed a guinea pig's head. At pulse widths less than 30 μs, thresholds were related to the density of absorbed energy (and to the density of incident energy). The minimal absorbed energy density per pulse is 5 mJ kg−1. As the width of the pulse increased, the threshold energy increased. For pulses longer than 70 μs, thresholds were related to the peak of the incident power density. The maximal power density of incident radiation is 90 mW cm−2 in the circular waveguide. The dependence of the evoked response on the width of the microwave pulse is in excellent agreement with predictions of thermal-expansion theory. These results provide more evidence that the microwave auditory effect is caused by a thermal expansion in the exposed head.