Precursor forms of peptide hormones may be biologically active with effects distinct from the mature end-product. Non-amidated progastrin-derived peptides stimulate growth of colonic epithelium, and are elevated in the circulation of patients with colorectal carcinomas while the amidated end product is the major regulator of gastric acidity. Using region specific radioimmunoassays, we here compared the in vitro and in vivo metabolism of recombinant human progastrin_6-80 and two other non-amidated gastrins: gastrin₁₇gly and tyr₇₀-progastrin_71-80. Although progastrin_6-80 was very stable in vitro, both progastrin_6-80 and gastrin₁₇gly were degraded in vivo. The in vivo data was best fitted by a double exponential decay curve, and the half-lives for progastrin_6-80 (t_1/2 = 5.1 ± 1.1, t_1/2ß = 42 ± 11 min) were significantly (P < 0.05) longer than for gastrin₁₇gly (t_1/2 = 2.2 ± 0.6, t_1/2ß = 13 ± 1 min). Tyr₇₀-progastrin_71-80 was degraded more rapidly. Comparison with amidated gastrins suggests that peptide length rather than sequence is the critical determinant of clearance. Progastrin has the clearance characteristics to be considered a circulating hormone.