The effect of isoamylethyl barbituric acid (amytal), in anesthetic, sedative and lethal doses, upon the pulse rate of the rat was determined by an auscultatory method. With all dosages used the pulse rate was increased. In general, the anesthetic doses caused a greater increase than did the sedative, but no strict correlation was found. Rats kept completely anesthetic by repeated doses of the drug over a period of four to eight hours showed an average heart rate for the entire period of 19 to 30 per cent above basal. With lethal doses the rate remained above basal until death occurred. The heart remained markedly labile, responding well to changes in temperature and to external stimuli.