Self-generating foamed fluids for circulation through a well during drilling operations and methods for circulating such self-generating foamed drilling fluids are provided for. The drilling fluid comprises an aqueous solvent, a polymer, a surfactant, a first foam generating agent, and a second foam generating agent. The first and second foam generating agents react to generate a gas within the drilling fluid and thereby to foam the drilling fluid. Preferably, the foam generating agents are selected such that they react to generate carbon dioxide. Preferably a base fluid comprising excess amounts of only one of the foam generating agents is pumped into the drill string using a hydraulic pump. The other foam generating agent then is injected in a controlled manner downstream of the circulation pump such that the drilling fluid is foamed after being pumped for circulation through the well. The novel drilling fluids preferably have a density of from about 5.8 lb/gal to about 0.83 lb/gal, and most preferably, from about 3.3 lb/gal to about 0.83 lb/gal.