The new experimental test facility was designed in order to generate an experimental database for pressure drop and flow regime maps in mini- and micro-channels. The test facility was equipped with state-of-the-art flow meters and differential pressure transducers. Once the test facility was built, three different circular test sections, with diameters of 3 mm, 1 mm, and 800 om, were used to study the two-phase frictional pressure drop and flow regime transition regions. Most of the experiments for each test section were executed at high local superficial gas velocities (U GS > 10 m/s).  The experimental pressure drop data was compared with the homogenous model, the Friedel (1979) model, and the Chisholm (1967) model. The homogenous model showed the most similarities with the data acquired during the course of this experiment, with an average error of 1.2%, 15.6%, and 24.1% for the 3 mm, 1 mm, and 0.8 mm test sections, respectively. However, it was also observed that the standard deviation of the errors increased as the channel diameter decreased. The Friedel (1979) model over-predicted the pressure drop for every test section, while the Chisholm (1967) model mostly underpredicted the pressure drop data.  The flow regimes were observed for high gas superficial velocities (U GS Y 10 m/s). There were three flow regimes and two transition regions observed in the 3 mm test section, which were in fairly good agreement with the experimental data obtained by Damianides (1987). There were two flow regimes and one transition region observed in both the 1 mm and the 0.8 mm test sections. Finally, the transition region from the annular to the dispersed flow regime in the 0.8 mm test section occurred when the superficial water velocity was approximately 0.9 m/s, which coincides with the results obtained in the 1 mm test tube. A 3CCD analog camera was used for the visualization of images in the present study. (Abstract shortened by UMI.)