Glucagon-like peptide 1 (GLP-1) has great potential in diabetes therapy due to its glucose-dependent stimulation of insulin secretion, but this is limited by its rapid degradation, primarily by dipeptidyl peptidase IV. Four analogues, N-terminally substituted with threonine, glycine, serine or alpha-aminoisobutyric acid, were synthesised and tested for metabolic stability. All were more resistant to dipeptidyl peptidase IV in porcine plasma in vitro, ranging from a t1/2 of 159 min (Gly8 analogue) to undetectable degradation after 6 h (Aib8 analogue; t1/2 for GLP-1 (7-36) amide, 28 min). During i. v. infusion in anaesthetised pigs, over 50% of each analogue remained undegraded compared to 22.7 % for GLP-1 (7-36) amide. In vivo, analogues had longer N-terminal t1/2 (intact peptides: means, 3.3-3.9 min) than GLP-1 (7-36) amide (0.9 min; p < 0.01), but these did not exceed the C-terminal t1/2 (intact plus metabolite: analogues, 3.5-4.4 min; GLP-1 (7-36) amide, 4.1 min). Analogues were assessed for receptor binding using a cell line expressing the cloned receptor, and for ability to stimulate insulin or inhibit glucagon secretion from the isolated perfused porcine pancreas. All bound to the receptor, but only the Aib8 and Gly8 analogues had similar affinities to GLP-1 (7-36) amide (IC50; Aib8=0.45 nmol/l; Gly8=2.8 nmol/l; GLP-1 (7-36) amide=0.78 nmol/l). All analogues were active in the isolated pancreas, with the potency order reflecting receptor affinities (Aib8 > Gly8 > Ser8 > Thr8). N-terminal modification of GLP-1 confers resistance to dipeptidyl peptidase IV degradation. Such analogues are biologically active and have prolonged metabolic stability in vivo, which, if associated with greater potency and duration of action, may help to realise the potential of GLP-1 in diabetes therapy.