Rats were irradiated with circularly polarized, 2,450-MHz pulsed microwaves ( 2 -p ~ pulses, 500 pulses per second [pps]) for 45 min in the cylindrical waveguide system of Guy et al: (Radio Sci 14:63-74, 1979). Immediately after exposure, sodium-dependent highaffinity choline uptake, an indicator of cholinergic activity in neural tissue, was measured in the striatum, frontal cortex, hippocampus, and hypothalamus. The power density was set to give average whole-body specific absorption rates (SAR) of 0.3, 0.45, 0.6, 0.75, 0.9, or I .2 W/kg to study the dose-response relationship between the rate of microwave energy absorption and cholinergic activity in the different areas of the brain. Decrease in choline uptake was observed in the striatum at a SAR of 0.75 Wlkg and above, whereas for the frontal cortex and hippocampus, decreases in choline uptake were observed at a SAR of 0.45 W/kg and above. No significant effect was observed in the hypothalamus at the irradiation power densities studied. The probit analysis was used to determine the SARSO in each brain area, i.e., the SAR at which 50% of maximum response was elicited. SARSO values for the striatum, frontal cortex, and hippocampus were 0.65, 0.38, and 0.44 W/kg, respectively.