A C-nitroso compound having a molecular weight ranging from 225 to 1,000 (from 225 to 600 for oral administration) on a monomeric basis wherein a nitroso group is attached to a tertiary carbon, which is obtained by nitrosylation of a carbon acid having a pKa less than about 25, is useful as an NO donor. When the compound is obtained from a carbon acid with a pKa less than about 10, it provides vascular relaxing effect when used at micromolar concentrations and this activity is potentiated by glutathione to be obtained at nanomolar concentrations. When the compound is obtained from a carbon acid with a pKa ranging from about 15 to 20, vascular relaxing effect is obtained at nanomolar concentrations without glutathione. In another embodiment, a biocompatible polymer incorporates a C-nitroso moiety.