Nitric oxide, thought to be the endothelium-derived relaxing factor (EDRF), is involved in intra- and intercellular signalling in various tissues. A system for the continuous detection of NO in the picomolar range from a perfused organ is described. The detection is based upon the chemiluminescence reaction between NO and the luminol (5-amino-2,3-dihydro-1,4-phthalazinedione)-H2O2 system. The chemiluminescence is due to the formation of peroxynitrite from NO and H2O2. The luminol-H2O2 system is specifically reactive to NO, so that other nitrogen-containing compounds, (organic nitrite, organic nitrate, and thio-nitroso compounds) or endothelium-derived compounds do not interfere. The limit of determination was approximately 100 fM. This system has been used to measure continuous NO release from isolated perfused rat kidney and the simultaneous changes of perfusion pressure. In Wistar rats basal NO release was 85 +/- 11 fmol/min-1 (g of kidney weight)-1 (39 pM in the perfusate), and acetylcholine increased NO release dose dependently with a concomitant pressure reduction. The changes in NO release were always associated with mirror image changes in the perfusion pressure. Simple pressure reduction did not interfere with the chemiluminescence. Precise titration data as well as results of some preliminary experiments using this method are presented.