The objective of this study is to evaluate the rheological properties (under HPHT conditions as well as ambient conditions) and flow characteristics of lightweight drilling fluids having densities less than that of the base fluids. This study concentrates on use of microsphere glass additives asdensity reducing agents. Four different types of glass microsphere additives with different compressive strengths (5000-18000 psi) and specific gravities (0.38-0.60) are being tested for their density reduction potential. Experiments are categorized in two groups: Rheology tests using a Fann75 HPHT viscometer and flow characterization tests using a flow loop. Experimental results show: 1) Test materials reduce the base fluid density successfully; 2) Average survival ratio is calculated as of 93% for all test materials under high pressure; 3) Rheological behavior of fluids containing test materials depend, not only on the test material volume concentration, but also on the composition of the base fluid used. Change in apparent viscosity with increasing concentrations of microspheres can be described with extended form of Einstein's equation for relative viscosity. 4) Pipe flow experiments suggest that viscosity increase versus concentration of microspheres is less than the increase measured onFann75 viscometers. A relationship between Fann75 measurements and pipe flow measurements is found for experimental results.