Cardiac output is traditionally measured by determining the dilution and washout of concentrated dye, hot/cold fluid or comparable indicator bolus-injected directed into the blood stream. The dilution response to such indicators has been shown to be linear, repeated, and mathematically very well represented by conventional linear system theory. We show here that the average cardiac output is inversely proportional to the time-integral of the so-called "impulse response" curve of the indicator-dilution system. The impulse response in general can be measured by performing a cross-correlation between the system input (indicator quantity) and the system output (blood stream concentration). This is considerably facilitated with mathematically well behaved input, namely, a repeating pseudorandom binary sequence. The impulse response (and therefore the average flow) of the thermal indicator-blood dilution system can thus be determined by continuously cross-correlating a pseudorandom binary (on-off) input injection of cold saline with the resulting small temperature fluctuation measured downstream. The flow so derived yields a continuous assessment of cardiac output. This theory will be experimentally tested first in the right-heart hydraulic fluid model and then in the right heart of a dog preparation. The results will be compared over a wide range of flows with those determined simultaneously by a number of alternate techniques. A low-power intracardiac heating element mounted on a flow-directed thermistor-tipped pulmonary artery catheter will also be developed and tested in both models as an alternative means of generating a pseudorandum binary thermal input. Continuous cardiac output will in this case be determined by cross-correlation of heater input power and pulmonary artery temperature.