Pressure drop measurements were made with a differential pressure transducer in the fully developed region of a horizontal circular straight tube with reentrant, square-edged, and bell-mouth inlets under isothermal and nonisothermal (uniform wall heat flux) flow conditions. The inlet Reynolds number for the ethylene glycol-water mixtures throughout the experiments ranged from about 1000 to 17,000 to cover laminar, transition, and turbulent regimes. The isothermal fully developed friction factors showed that the range of Reynolds number values for which transition flow exists is about 2900-3500 for the reentrant inlet, 3100-3700 for the square-edged inlet, and 5100-6100 for the bell-mouth inlet. Different heat fluxes (3, 8, and 16 kW/m2) were applied to the test section to investigate the effect of heating on the friction factor. The results indicated that the value of the fully developed friction factor increased with an increase in the heating rate for a fixed Reynolds number. Owing to the presence of secondary flow, the effect of heating on the friction factor was significant in the laminar and transition regions. This increase in friction factor caused an increase in the lower and upper limits of the isothermal transition boundaries. For example, for the 16 kW/m2 heat flux, the transition boundaries increased to about 4100-5900 for the reentrant inlet, 4500-6400 for the square-edged inlet, and 7300-9600 for the bell-mouth inlet. Available correlations for the prediction of nonisothermal fully developed friction factors are compared with our experimental data. Correlations for the prediction of the nonisothermal fully developed friction factors in the laminar and transition regions for the three inlets are recommended. The effect of heating in these correlations was accounted for in terms of a bulk-to-wall-viscosity ratio expressed as a function of Prandtl and Grashof numbers. © Elsevier Science Inc., 1997.