Metabolism of bovine 125I-labeled parathyroid hormone was studied in the rat by gel filtration and by sequence analysis of the iodinated fragments. Analysis of the kinetics of hormone metabolism shows that iodinated intact hormone has a multiexponential disappearance curve with a rapid (3 min) initial and a slower (48 min) 2nd component. Iodinated fragments, which rapidly increase during the first 12 min after injection of the intact hormone, subsequently disappear from the circulation with a t1/2 [half-time] of no greater than 48 min. Plasma samples collected at various time-intervals after i.v. injection of bovine 125I-labeled parathyroid hormone were gel filtered on Bio-gel P-100. Four radioactive peaks were seen. The 1st and 2nd peaks eluted, respectively, at the void volume of the column and at the position of intact hormone. The 3rd peak consisted of iodinated fragments, and the last peak eluted at the salt volume of the column. Sequence analysis of the iodinated fragments in the 3rd peak showed that it was heterogeneous, containing several different, but closely related, polypeptides. Before 48 min after injection, the most-abundant fragment is one whose amino-terminal amino acid is residue 34. The amino-terminal residue of the next most-common fragment is the amino acid at position 37. No fragments representing cleavages closer to the amino-terminus than residue 34 were seen. The results of these studies are virtually identical with those previously obtained in the dog. The similarities found in the sites of hormone proteolysis and in the kinetics of hormone metabolism in the rat and dog, coupled with the less direct evidence indicating that similar cleavages are present in man and bovine, are consistent with the view that proteolysis of parathyroid hormone in peripheral tissues is specific, at least in mammalian species, and may be a critical step in controlling the availability of biologically active hormone.