A brief review of the studies on the obese (ob) gene is given. The ob gene is a mouse gene, the mutations of which are associated with altered metabolism and increased lipid deposits in adipose tissue. Recessive ob gene mutations in homozygous mice result in obesity and diabetes mellitus. Both mouse and human ob cDNAs were cloned and sequenced using positional cloning, exon trapping, and PCR. Of ten tested tissues, the ob gene was expressed only in white adipose tissue. The ob gene cDNA has a region of the nucleotide sequence with an opening reading frame and encodes the ob protein consisting of 167 amino acid residues. Mouse and human ob proteins showed a 85% homology. The 145-amino acid peptide termed as leptin and derived from ob protein after cleavage of signal peptide is secreted in the blood and stimulates fat consumption in energy metabolism. The biologically active ob peptide has been obtained by gene engineering methods. Administration of the ob protein to ob/ob mice reduced body weight and abolished symptoms of diabetes. The ob protein lowered body weight also in healthy animals. It was biologically effective both upon parenteral and intravenous administration and also when injected into lateral ventricle of the brain. With a polyclonal antiserum against the peptide the ob protein was shown to be present in human and mouse plasma and mouse adipose tissue. Based on the data obtained, it is postulated that the ob gene protein product leptin, is a hormone, which is secreted by adipocytes in the blood in varying amounts and regulates the mass of adipose tissue by stimulating lipid metabolism. Similarly to adipocytes, many other organs and tissues are presumably endocrine and may secrete peptide hormones in the blood. This considerably extends the scope of endocrinology and makes it necessary to review the existing concepts and views.