Abstract
1. The abilities of some pyrimidopyrimidines (including dipyridamole) and pteridine derivatives to potentiate heart block produced by adenosine in the guinea-pig and to produce coronary dilatation in kitten hearts were investigated.2. The results demonstrated a correlation between adenosine potentiating and coronary dilating activities of the compounds studied, and are therefore compatible with the concept that compounds of the dipyridamole type owe their coronary dilator activity to potentiation of endogenous adenosine.