This paper describes a microfluidic system in which fluids are pumped by centrifugal force through microscopic channels defined in a plastic disk in order to perform complex analytical processes. The channels are created either by casting poly(dimethylsiloxane) against molds fabricated by photolithography or by conventional machining of poly(methyl methyacrylate). The channels have a wide range of diameters (5 μm−0.5 mm) and depths (16 μm−3 mm). Fluids are loaded into reservoirs near the center of the disk, the disk is rotated on the shaft of a simple motor at 60−3000 rpm, and the fluids are pumped outward by centrifugal force through microfluidic networks. The control of flow in the time domain, i.e., gating, is achieved by the use of passive valves based on capillary forces. Flow rates ranging from 5 nL/s to >0.1 mL/s have been achieved using channels of different dimensions and different rates of rotation. The method of pumping is insensitive to many physicochemical properties of the liquid, such as pH and ionic strength, so it has been possible to pump biological fluids, such as blood and urine, a buffer containing a detergent, and some organic solvents. A system that performs multiple (48) enzymatic assays simultaneously using colorimetric detection on a dedicated instrument has been demonstrated. These integrated assays have been used both to yield the Michaelis constant (K_m) of an enzyme and to determine the dose response of an enzyme to a drug. The fluid pumping and control embodied in this system may be readily integrated with other analytical components (e.g., heating, detection, and informatics) to form the basis for a microscale total analysis system for use in genomics, proteomics, high-throughput screening, and molecular diagnostics.