The uptake of cyclopropane was measured in 5 human subjects who breathed a 1% concentration of the gas in oxygen for periods ranging from 7 to 62 minutes. The measured concentrations of cyclopropane in end-expired air were compared with those predicted by Kety's equation (Pharmacol. Rev. 3:1, 1951) describing the saturation process for inert gases. Although fair agreement was found between the measured and predicted data, it was noted that the measured values increased more rapidly than theory predicted. This difference apparently resulted from Kety's simplifying assumption that the body consists of a homogeneous tissue mass. Submitted on May 25, 1959