The carotenoid compound crocetin has been hypothesized to enhance the diffusion of O₂ through plasma, and observations in the rat and rabbit have revealed improvement in arterial PO₂ when crocetin is given. To determine whether crocetin enhances diffusion of O₂ between alveolar gas and the red blood cell in the pulmonary capillary in vivo, five foxhounds, two previously subjected to sham and three to actual lobectomy or pneumonectomy, were studied while breathing 14% O₂ at rest and during moderate and heavy exercise before and within 10 min after injection of a single dose of crocetin as the trans isomer of sodium crocetinate (TSC) at 100 µg/kg iv. This dose is equivalent to that used in previous studies and would yield an initial plasma concentration of 0.7-1.0 µg/ml. Ventilation-perfusion inequality and pulmonary diffusion limitation were assessed by the multiple inert gas elimination technique in concert with conventional measurements of arterial and mixed venous O₂ and CO₂. TSC had no effect on ventilation, cardiac output, O₂ consumption, arterial PO₂/saturation, or pulmonary O₂ diffusing capacity. There were minor reductions in ventilation-perfusion mismatching (logarithm of the standard deviation of perfusion fell from 0.48 to 0.43, P = 0.001) and in CO₂ output and respiratory exchange ratio (P = 0.05), which may have been due to TSC or to persisting effects of the first exercise bout. Spectrophotometry revealed that TSC disappeared from plasma with a half time of ~10 min. We conclude that, in this model of extensive pulmonary O₂ diffusion limitation, TSC as given has no effect on O₂ exchange or transport. Whether the original hypothesis is invalid, the dose of TSC was too low, or plasma diffusion of O₂ is not rate limiting without TSC cannot be discerned from the present study.