Fig. 5. Line graphs showing stimulus–response functions for mean withdrawal forces elicited by graded noxious heat under halothane (A -C ; n = 6) or isoflurane (D -F ; n = 9) anesthesia. Mean peak withdrawal forces are shown for halothane (A ) and isoflurane (B ). Mean integrated forces generated during the 10-s heat stimulus are shown for halothane (B ) and isoflurane (E ), and integrated forces generated during 30-s following the onset of heat are shown in C  and F , for halothane and isoflurane, respectively. Increases in concentration of either anesthetic were accompanied by increases in withdrawal thresholds (P < 0.001) and large reductions in, or absence of, withdrawal responses at 1.1 of the minimum alveolar concentration (MAC), compared to 0.9 MAC (P < 0.015). Mean withdrawal force was greater under isoflurane compared to halothane anesthesia (P < 0.02). Error bars = SEM.