This study was designed to assess the relationship between MAC and hypertension. To this purpose, MAC of halothane was determined in fully inbred spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). Because MAC determination was performed in animals whose lungs were mechanically ventilated, the adequacy of the ventilation was initially established in 20 rats equally divided into SHR and WKY, and instrumented with catheters in the abdominal aorta. Subsequently, MAC of halothane was determined in 40 rats equally divided into SHR and WKY, including those instrumented. There were no differences in MAC of halothane between SHR (n = 20) and WKY (n = 20) (1.08 +/- 0.02% vs. 1.11 +/- 0.02%). Subgroup analysis indicated that MAC of halothane was not affected by the presence of an arterial catheter in the abdominal aorta (SHR 1.09 +/- 0.06% vs. 1.08 +/- 0.02%; WKY 1.15 +/- 0.04% vs. 1.08 +/- 0.02%). The authors' data provide experimental evidence that MAC is not affected by either chronic hypertension or limited instrumentation.