To the Editor: The letter by Palmer and Lassiter in the June 17 issue of the Journal bases a warning about clinical use of nitroprusside in the treatment of dissecting aneurysm on studies carried out in the anesthetized dog. The authors appear to have demonstrated an increase in myocardial contractility and in the rate of rise of aortic pressure in response to nitroprusside. These effects could be deleterious in the setting of aortic dissection. Although heart rates were not reported in their letter, it has been our experience that nitroprusside produces considerable tachycardia in dogs but not in man. No extract is available for articles shorter than 400 words.