Y4 receptor agonist peptide selected from the group consisting of: [Ala30]PP2-36, [Thr30]PP2-36, [Asn30]PP2-36, [Gln30]PP2-36, [Glu10]PP2-36, [Glu10,Leu17,Thr30]PP2-36, [Nle17,Nle30]PP2-36, [Glu10,Nle17,Nle30]PP2-36, their PP1-36 equivalents, and analogues and derivatives thereof as described in the specification, are selective agonists of the Y4 receptor relative to the Y1 and Y2 receptors, and are useful in the treatment, for example, of obesity and overweight, and conditions in which these are considered contributory factors, and in the treatment of diarrhoea and intestinal hypersecretion.