Using a specific and sensitive radio-immunoassay involving separation and extraction procedures, we measured the concentration of saliva and plasma atrial natriuric factor (ANF) and cyclic guanosine monophosphate (cGMP) in men before and during a 10 h head-down tilt at -6 degrees. Saliva values closely correlated with plasma for ANF (r = 0.7-0.95) and for cGMP (r = 0.65-1). During this dynamic test, the mean concentrations of ANF and cGMP were significantly higher after 15 and 45 min, respectively, this increase persisting for 3.5 h. We concluded that the concentration of ANF in saliva may be significantly affected by a marked fluid shift from the lower to the upper half of the body. This is the first time that the presence of ANF and cGMP has been demonstrated in saliva. The great advantage of studying saliva is that it can be obtained non-invasively in athletes or during space flight. This methodology will be used during the Soviet-French space flight (Antares Project) planned for 1992.