The effects of morphine, halothane, and their various combinations on the purposeful movement (PM) response and the heart rate (HR) increase caused by noxious stimulation were studied in 250 rat experiments. Doses that block the PM and HR responses for the single agent and for combinations were determined with a probit procedure and compared with an isobolographic analysis. As was evidenced by the PM response, the combined anesthetic effect of morphine and halothane, with some deviations, may be defined as additive. It also was found that the combined administration of morphine and halothane results in an antagonism for suppression of the HR increase to noxious stimulation. Halothane antagonized morphine to a much greater extent than morphine to halothane.