Leptin is a 167 amino acid hormone produced chiefly by adipocytes. It plays an important role in regulation of food intake, energy metabolism and reproduction in mammals. The DNA sequence of a chicken leptin gene has been reported which shares 95% nucleotide similarity with the mouse leptin coding region (Taouis et al, 1998, Gene 208:239-242). This has been confirmed by demonstrating the expression of a leptin homologue in liver and adipose tissue from the broiler chicken which is regulated by insulin, glucagon, dexamethasone and oestrogen [Ashwell et al, 1999 Am. J. Physiol. 276 (Regulatory Integrative Comp. Physiol. 45): R226-R232]. Experiments were carried out to confirm these findings using RT-PCR, Southern analysis, screening of a chicken genomic cosmid library and immunocytochemistry. Seven different PCR primer sets were designed to amplify the chicken leptin gene based on mouse and chicken leptin sequences, with two of them being identical in both species. PCR amplification was carried out on genomic DAN (gDNA) and reverse-transcribed mRNA from the fat, liver and pancreas of several chicken strains. No PCR products sharing close similarity to the mouse leptin sequence were generated from any chicken templates whereas amplification of mouse leptin sequence was consistently obtained from control mouse templates. Southern blots under low stringency hybridisation and washing conditions revealed hybridisation of a mouse leptin probe to chicken gDNA. With higher stringency washing the chicken signals disappeared while those from control mouse and sheep gDNA remained. Screening of chicken genomic cosmid library using the same mouse probe failed to pick up any closely related sequence. No immunocytochemical labelling was observed in chicken adipose tissue or liver using antibodies (Santa Cruz) against highly conserved N and C terminal regions of human leptin. Control mouse adipose tissue, but not liver, showed strong immunocytochemical labelling. These data indicate that a leptin-like sequence exists in the chicken genome which is likely to be less than 75% homologous to the mouse leptin gene.