Compared to conventional (incompressible) drilling fluids, relatively little is known about the hydraulic and rheological properties of foamed drilling fluids. The complex flow mechanisms involved in compressible drilling fluid circulation make determination of the optimum combination of liquid and gas injection rates very difficult. Modelling of foam rheology is the key issue in hydraulic design, in order to predict the bottom-hole preesure accurately, and to optimize the different controllable variables for effective cutting transport performance. The University of Tulsa's low-pressure ambient temperature flow loop has been recently modified to accommodate foam. The flow loop permits foam flow through 0.0508 m (2 in.), in 0.0762, (3 in.) 0.1016 m (4 in.) diameter pipes, and a 0.2032 m (8 in.) by 0.1143 m (41/2 in.) annular section. Preliminary experiments have been conducted, in which pressure losses were measured for different foam qualities. Measured parameters were gas/liquid flow rates, pressure, differential pressure loss, and temperature. Statistical analysis was carried out to see the degree of fit provided by Bingham plastic, power law, and yield power law models for the generalized foam flow curve data. A comparative study was conducted to investigate the predictive ability of the available foam hydraulic models. Models presented by Beyer et al. (1972), Blauer et al. (1974), Reidenbach et al. (1986), Sanghani and Ikoku (1983), Gardiner et al. (1988) and Valko and Economides (1992) were used to estimate the frictional pressure losses during the flow of foam in horizontal pipes. Comparison of the model predictions with experimental pressure loss values show that model predictions of frictional pressure losses can be different from the actual values by 2 to 250 %.