Ghrelin is a 28 amino acid stomach peptide, derived from proghrelin(1-94), that stimulates GH release, appetite and adipose deposition. Recently, a peptide derived from proghrelin(53-75) -- also known as obestatin -- has been reported to be a physiological antagonist of ghrelin in the rat. Using four specific RIAs, we provide the first characterisation of proghrelin(1-94) peptides in human plasma, their modulation by metabolic manipulation and their distribution in mammalian tissues. ghrelin(1-28) immunoreactivity (IR) in human plasma and rat plasma/stomach consisted of major des-octanoyl and minor octanoylated forms, as determined by HPLC/RIA. Human plasma ghrelin(1-28) IR was significantly suppressed by food intake, oral glucose and 1 mg s.c. glucagon administration. ghrelin(1-28) IR and proghrelin(29-94) IR peptide distributions in the rat indicated that the stomach and gastrointestinal tract contain the highest amounts of the peptides. Human and rat plasma and rat stomach extracts contained a major IR peak of proghrelin(29-94)-like peptide as determined by HPLC/RIA, whereas no obestatin IR was observed. Human plasma proghrelin(29-94)-like IR positively correlated with ghrelin(1-28) IR, was significantly suppressed by food intake and oral glucose and shared with ghrelin(1-28) IR a negative correlation with body mass index. We found no evidence for the existence of obestatin as a unique, endogenous peptide. Rather, our data suggest that circulating and stored peptides derived from the carboxyl terminal of proghrelin (C-ghrelin) are consistent in length with proghrelin(29-94) and respond to metabolic manipulation, at least in man, in similar fashion to ghrelin(1-28).