Somatostatin (somatotropin release-inhibiting factor, SRIF) is produced throughout the body in the form of 14- or 28-amino acid peptide, and both peptides inhibit the release of numerous secondary hormones by binding to G-protein-coupled SRIF receptor (sst) subtypes 1-5 (sst1 to sst5) with high affinity. Native SRIF has only limited clinical applications due to the short duration of action in the blood (half-life < 3 min). To overcome this obstacle, synthetic SRIF analogs such as octreotide were developed. However, octreotide, an sst2 specific ligand, encounters a difficulty in treating octreotide-resistant pituitary adenomas or non-growth hormone adenomas where sst2 is not expressed. Therefore, if we are able to develop an SRIF analog that can be not only metabolically stable but can also target all of the five receptor subtypes, this should offer a useful alternative to octreotide for clinical applications. In the present study, we synthesized glycosylated SRIF analogs by attaching one to four complex type sialyloligosaccharide chain(s) to the SRIF-28 molecule. These analogs were found to obtain a prolonged half-life by increasing the number of attached glycans and show a high affinity to all the subtypes as in the case of native SRIF. These results clearly indicate that glycosylated SRIF analogs could be a potent therapeutic drug for acromegaly, octreotide-resistant diseases and adenomas/tumors where several receptor subtypes are closely involved.