Components of the autonomic control of the heart were studied in 5 young normals subjected to: 1) sustained lung inflation (SLI) in various postures; (2) mental loading by concentration of attention, 3) face immersion in cold water under SLI or snorkel breathing, when sitting; 4) superposition of above experimental interventions. We recorded cardiotachogram (RR interval) and T-amplitude from ECG, ear lobe photoplethysmogram, digital impedance rheogram, thermistor respiration. Cardiac data were explored by means of Fourier spectral analysis on a M 118 B microcomputer. Results caution that allegedly unilateral stimulation by some "vagal" maneuvers triggers in fact amphotropic heart rate control menus involving sympathetic nonreciprocal response. Subsequent bradycardia should be interpreted as an absolute indicator of the parasympathetic function only when a separate measure of the sympathetic counterpart is available. In vigil young adult at physical rest the medullary cardiovascular centers appear as subjected to "sliding capture" by their prominent input signal--as determined by various receptor stimulation or cortical status, the latter preserving a higher priority at competing task demands. In a coupled study 9 normals were polygraphically monitored (EEG, EOG, ear duct temperature and the above variables as well) during 2 nonconsecutive nights and cardiac data were spectrally examined along various sleep stages. Results suggest an increased dual autonomic tone during REM that could be related to reported stage-specific arrhythmias. Reciprocal dynamics of the respiratory versus lower frequency variability of RR interval from NREM to REM sleep points to a useful discriminant for computer sleep stage scoring.