The extent of the destruction, during passage through the lungs, of a substance secreted by the heart.

1. In vitro, the lungs of male Wistar rats, 220-230 g, removed heart substance (HS) from saline perfusates. HS could not be recovered by subsequent perfusion with HS free solution.2. A maximum rate of HS-uptake, 8.5 ng biological equivalents of 18-monoacetate of D-aldosterone (18 MA)/min, was reached at a concentration of HS equivalent to 3.5 ng 18 MA/ml. at a flow of 6 ml./min.3. Uptake was not significantly affected by a fall in temperature from 38 to 18-20 degrees C or by reduction in P(CO) (2) to zero. Uptake was reduced by anoxia and was enhanced by decrease of pH from 7.4 to 6.6.4. 30% of the HS taken up by the lungs at 20 degrees C was recoverable: none of the HS taken up at 38 degrees C was recovered.5. Uptake of HS by binding is considered rate-limiting to the destruction of HS in the lungs.6. The concentrations of HS found in the pulmonary and carotid arterial blood of cats under chloralose anaesthesia, during haemorrhage, were biologically equivalent to 1098 +/- 38.7 and 169 +/- 13.4 ng 18 MA/100 ml. (means +/- S.E.) respectively, in four experiments.