Pt-disk electrodes (50 mum diameter) were modified with pyrrole-functionalised Mn(II) trimethoxycarboxyphenylporphyrin by means of an electrochemical pulse technique leading to stable and sensitive microsensors that displayed electrocatalytic activitiy suitable for the detection and quantification of nitric oxide (NO). In order to improve selectivity, a thin layer of Nafion(R) was deposited on top of the porphyrin-modified surface of the microsensor by means of a dip-coating procedure. These microelectrodes were successfully applied for the detection of the secretion of NO from a population of human umbilical vein endothelial cells (HUVEC) into a small volume of electrolyte. (C) 2003 Elsevier Ltd. All rights reserved.