In this article a detailed study on the effects of different inlet geometries and heating on the friction factor in the entrance and fully developed regions of a horizontal tube under different flow regimes is presented. For this purpose, accurate pressure drop measurements were made in the entrance and fully-developed regions of a 1.48. cm inside diameter horizontal circular straight tube with square-edged and re-entrant inlets under isothermal and non-isothermal (uniform wall heat flux) boundary conditions. The working fluid used in the experiments was different mixtures of ethylene glycol and water and the Reynolds number for the experiments ranged from about 800 to 22,000 to cover laminar, transition, and turbulent regimes. Due to the presence of secondary flow, the effect of heating on the friction factor was significant in the laminar and transition regions. The heating condition caused an increase in the lower and upper limits of the isothermal transition boundaries and a decrease in the entrance and fully developed flow apparent friction factors in the laminar and transition regions. Available correlations for the prediction of non-isothermal fully developed friction factors were compared with our experimental data. Owing to lack of the non-isothermal entrance flow correlations in the laminar and transition regions, correlations for prediction of the non-isothermal entrance and fully-developed friction factors in these flow regimes for the square-edged and re-entrant inlets were developed. © 2012 Elsevier Inc.