Bispectral analysis, introduced recently as a technique for revealing time-phase relationships among interacting noisy oscillators, has been used to study the nature of the coupling between cardiac and respiratory activity. Univariate blood flow signals recorded simultaneously by laser-Doppler flowmetry (LDF) on both legs and arms were analysed. Coupling between cardiac and respiratory activity was also checked by use of bivariate data and computation of the cross-bispectrum between simultaneously recorded ECG, blood pressure and respiratory signals. Recordings were taken during spontaneous breathing and paced respiration. It is shown that cardiorespiratory coupling exists during spontaneous as well as during paced respiration and that coupling information among cardiac and respiratory processes in young, healthy subjects is spatially invariant, but can be expected to change with cardiovascular diseases.