A novel electrochemical microsensor for the determination of NO based on an electropolymerized film of tetraaminophthalocyaninecopper [Cu(TAPc)] was prepared. Its response to NO and its application to the study of an NO donor (S-nitrosoglutathione; GSNO) are also described. The microsensor exhibited an electrocatalytic effect on NO oxidation and showed a low detection limit, high sensitivity and selectivity for NO determination. The oxidation current (measured by differential pulse amperometry) was linear for NO concentrations ranging from 6.2 × 10⁻⁹ to 3.0 × 10⁻⁵ mol L⁻¹ with a calculated detection limit of 4.0 ×10 ⁻⁹ mol L⁻¹ (S/N = 3) and a linear coefficient of 0.9984. Some endogenous electroactive substances in biological tissues, such as dopamine, 5-hydroxytryptamine and nitrite, at concentrations higher than those in biological systems did not interfere with NO determination. The sensor shows promise for the possible in vivo determination of NO. Using the microsensor, the NO release from the NO donor (GSNO) was successfully monitored. This work sets a foundation for the study of the pharmacology and the biological effects in vivo of S-nitrosothiols.