The objective of this research was to examine the time- and dose- dependent disturbances in the hypothalamic-pituitary-thyroid (HPT) axis of adult male rats administered a potent coplanar (non-ortho) PCB, 3,3',4,4',5-pentachlorobiphenyl (PCB 126). Adult male Sprague-Dawley rats were administered a single oral bolus dose of 0, 7.5, 75, or 275 microg PCB 126/kg bw dissolved in corn oil. The rats were sacrificed periodically over 22 days. The 7.5-microg/kg dose induced hepatic ethoxyresorufin-O-deethylation EROD activity, but no changes were observed in hepatic uridine diphosphate glucuronyl transferases (UDPGTs) activity or serum TSH, T4, or fT4 concentrations. The two highest doses caused a modest decline in weight gain, induced hepatic EROD and UDPGT activities, increased serum TSH concentrations, and decreased serum T4 and fT4 concentrations. The amount of thyroxine glucuronide formed daily (pM/mg protein) increased linearly with the area-under-the-concentration-curve (AUCC) for PCB 126 in liver (microg/kg/day) and then slowed at the 275-microg/kg PCB 126 dose. Perturbations in the HPT axis were nonlinear with respect to PCB 126 dosing. As expected, an inverse relationship between the AUCC for serum T4 (microg/dl/day) and the AUCC for serum TSH (ng/dl/day) was observed; however, the relationship was highly nonlinear. These data support a mode of action for PCB 126 involving induction of hepatic UDPGTs by the aryl hydrocarbon receptor AhR. However, the dose-response characteristics of the HPT axis are nonlinear and complex, requiring sophisticated tools, such as PBPK models, to characterize dose response.