This study presents the analysis and the evaluation of indoor daylighting levels of Ex G.I.L. building (1935) in Macerata (Italy), designed by Architect Mario Ridolfi (1904-1984), according to the original composition and internal distribution, which have been deeply modified during time. The Ex G.I.L. building is a very representative example of Italian rationalist architecture. The virtual reconstruction of the original aspect of the building has been possible thanks to the study of sources and of original documents. The simulations, produced to evaluate and to represent the levels of indoor natural light, have involved a very reliable 3D virtual reconstruction of the original building about size, building components dimensions and internal distributions, allowing at the same time to provide reliable and new representations of the original building. The daylighting simulations are run, using advanced solar-lighting design tools, which use international technical lighting calculations, according to ISO/CIE standards. Finally, the data and simulations obtained are compared and evaluated with current regulatory standards. The use of current energy simulation tools on the study of this architecture of the past, shows a very high availability of natural light and optimum light distribution of indoor environments designed by Ridolfi, highlighting a specifically design attention dedicated to this purpose, where the excellent formal solution, according to the rationalist style, includes the achievement of optimum natural lighting and visual comfort. (C) 2015 Elsevier B.V. All rights reserved.