The in-vivo half-lives of insulin and C-peptide have been assessed in normal man by a method which examines the decline of endogenously produced insulin and C-peptide after somatostatin suppression of secretion. Venous blood samples were taken each minute from seven normal subjects: i. v. glucose (0·1 g/kg ideal body weight) was given over 1 min to stimulate secretion, followed by a bolus of 250 fig of somatostatin-14 and an infusion of a further 250 jug somatostatin-14 over the subsequent 30 min. Plasma samples were analysed for C-peptide, glucose and insulin. The initial mono-exponential half-lives over 8 min were 3·9 · 0·3 and 10·2 · 0·7 min respectively (mean · SEM), with subsequent slower declines. Log transformed insulin and C-peptide yielded biphasic declinations which were assessed by a two-pool model. The rate constant of clearance of insulin implied avid uptake, while the kinetics of C-peptide clearance were slower, and irreversible loss might be explained by glomerular filtration alone. The somatostatin suppression method of measuring hormone kinetics could be used for newly described hormones which are not available for in-vivo studies.