The lack of a suitable objective yardstick to measure the effect of anti-anginal drugs has led to confusion in their clinical evaluation. A method of quantitating the nitrate level in the serum has furnished a means of correlating the subjective relief with this group of drugs. Representative samplings of serum from noncardiac and cardiac patients were analyzed to establish control nitrate levels. The cardiac patients were selected for their good responses to nitroglycerin and pentaerythritol tetranitrate. A good subjective response to the drugs correlated with the increased blood nitrate levels.T HE difficulties inherent in evaluating anti-anginal drugs have been stressed by a number of authors.