The effects of nucleosides and nucleotides on the tube formation of human umbilical vein endothelial cells (HUVEC) were compared. Twenty eight compounds including endogenous species were examined and many of them were revealed to have inhibitory potency. The potency was strengthened significantly when the sugar moiety of ribonucleosides was modified. In case that chlorine was introduced to the 2-position of adenine, the potency also increased. The structure-activity relationship was considered together with the effects on the proliferation of HUVEC.