The half-life in the circulation of 125I-labelled porcine neurophysins, injected intravenously into rats, was determined. The radioactivity in blood collected at intervals after injection was characterized as neurophysin by polyacrylamide gel electrophoresis. The half-life of neurophysin I was 1·5 ± 0·1 (s.e.m.) min and that of neurophysin II was 1·7 ± 0·1 min. The metabolic clearance rate of neurophysin I and neurophysin II was 1·9 ± 0·1 and 1·3 ± 0·2 ml/min/200 g respectively. Clearance from the circulation was due to both equilibration with the extravascular space and rapid specific accumulation by the kidney. The neurophysins were filtered at the glomerulus. Most of the neurophysin taken up by the kidney was incorporated into lysosomes, probably in the proximal tubule and degraded to lowmolecular-weight metabolites which appeared in urine and which represented the principal mode of excretion of neurophysins.