Leptin is a signalling factor involved in the regulation of body weight and is synthesised predominantly by adipocytes. In humans, there is a positive correlation between plasma concentration of leptin and body mass index (kg/m_a3) and subcutaneous fat (Considine et al., 1996; Lonnqvist et al., 1995). In vitro adipocytes obtained from women secrete more leptin than those of men (Casabeill et al., 1998). Furthermore, testosterone inhibits the expression of the leptin gene in the rat (Wu-Peng et al., 1999). The aim of this study was to examine whether gender, age and body conformation influenced plasma leptin and thyroid hormone concentrations in the horse. Materials and method Pre-slaughter body weight and height were recorded in a random group of mares (n=5), geldings (castrated males: n=7) and stallions (n=3) destined for human consumption. Their age was estimated by dental examination. Immediately post-mortem, a jugular vein blood sample was collected into a heparinised tube, which was centrifuged at 2500 rpm for 10 minutes. Plasma was stored in liquid nitrogen until analysed for plasma concentrations of leptin and thyroid hormones using human ELISA (DRG Instruments) and RIA kits (ICN Pharmaceuticals Ltd), respectively. Statistical differences between groups were assessed using General Linear Model, Analysis of Variance. Regression analysis was used to determine whether plasma leptin concentration was related to body conformation and age.