A previous study showed a substantial increase in the colonic temperature of rhesus monkeys (Macaca mulatta) exposed to radiofrequency (RF) fields at a frequency near whole-body resonance and specific absorption rates (SAR) of 2-3 W/kg. The present experiments were conducted to determine the metabolic and vasomotor responses during exposures to similar RF fields. We exposed five adult male rhesus monkeys to 225 MHz radiation (E orientation) in an anechoic chamber. Oxygen consumption and carbon dioxide production were measured before, during, and after RF exposure. Colonic, tail and leg skin temperatures were continuously monitored with RF-nonperturbing probes. The monkeys were irradiated at two carefully-controlled ambient temperatures, either cool (20 degrees C) or thermoneutral (26 degrees C). Power densities ranged from 0 (sham) to 10.0 mW/cm2 with an average whole-body SAR of 0.285 (W/kg)/(mW/cm2). We used two experimental protocols, each of which began with a 120-min pre-exposure equilibration period. One protocol involved repetitive 10-min RF exposures at successively higher power densities with a recovery period between exposures. In the second protocol, a 120-min RF exposure permitted the measurement of steady-state thermoregulatory responses. Metabolic and vasomotor adjustments in the rhesus monkey exposed to 225 MHz occurred during brief or sustained exposures at SARs at or above 1.4 W/kg. The SAR required to produce a given response varied with ambient temperature. Metabolic and vasomotor responses were coordinated effectively to produce a stable deep body temperature. The results show that the thermoregulatory response of the rhesus monkey to an RF exposure at a resonant frequency limits storage of heat in the body. However, substantial increases in colonic temperature were not prevented by such responses, even in a cool environment.