In this study we have investigated the effects of breath holding and of the physical properties of gases on four different respiratory dead spaces (VD): the Fowler, the physiological, the washout and the inert gas dead space. The experiments were performed with dogs which were ventilated artificially with breathing patterns with different post-inspiratory breath holding times (ta) of 0, 0.5, 1.0 and 2.0 s. Tracer amounts of acetone, ether and enflurane were infused continuously into a peripheral vein and a bolus of a mixture of krypton, Freon12 and SF6 was introduced into the peritoneal cavity. After reaching steady state, samples of arterial blood, mixed venous blood and mixed expired air were taken simultaneously. From the partial pressures (Pa, PV and PE respectively) we determined the excretion (E = PE/PV), retention (R = Pa/PV) and the physiological dead space fraction (VD,phys/VT = (1- PE/Pa)) for each gas, where VT is tidal volume. Further, we recorded the expirograms of the six tracer gases and of CO2 from which the Fowler dead space fractions (VD,Fowler/VT) of the different gases were determined. Also the washout dead space fractions (VD,washout/VT) for He and SF6 were determined as well as the inert gas dead space fraction (VD,MIGET/VT) with the use of the multiple inert gas elimination technique (MIGET). With the exception of VD,phys/VT for SF6, all dead space fractions decreased with increasing ta. VD,phys/VT for the poorly soluble gas SF6 was considerably larger than VD,phys/VT for the remaining gases. For the highly soluble acetone VFowler/VT was considerably smaller than VD,Fowler/VT for the other gases. VD,washout,SF6/VT was always larger than VD,washout,He/VT and VD,Fowler,SF6/VT. Further, VD,phys/VT was larger than VD,Fowler/VT for SF6 and acetone. However, for gases with intermediate solubility in blood VD,phys/VT tended to be smaller than VD,Fowler/VT. We conclude that the respiratory dead spaces are affected by the breathing pattern and by the physical properties of gases, i.e. their diffusivity in alveolar gas and their solubility in blood or lung tissue.