This paper presents local experimental in-cylinder concentrations of nitric oxide, obtained by laser-induced fluorescence measurements in a heavy-duty diesel engine. Quantitative concentration histories during the entire combustion stroke are shown for a number of fuel injection timings. Using images from high-speed combustion visualization experiments, the presence of the diffusion flame is related to the onset of NO formation within the laser probe volume. Further attention is paid to the possible NO formation mechanisms. Off-line characterization of the fuel sprays by means of Schlieren imaging reveals that the initial (premixed) combustion is too fuel-rich for thermal (Zeldovich) NO formation. Furthermore, the experimental NO concentrations are compared to numerical calculations of the thermal NO formation during the mixing-controlled combustion phase. The agreement between model and experiments suggests that the thermal mechanism is the major NO formation pathway. However, it cannot be excluded that transport to the probe volume of early NO, formed under conditions where the thermal mechanism is ineffective, might be of some importance as well. (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.