The aim of this paper is to determine the concentration of blood metabolic hormones and leptin levels in growing lambs. The research was carried out on Tsigai lambs in two periods (suckling and fattening) during the winter feeding season. Lambs were suckling and ate a food mixture and alfalfa hay ad libitum, while during the fattening period they were fed only with the above mentioned mixture and alfalfa hay ad libitum. Their blood was analyzed on 35th and 75th day of age. Concentrations of minsulin, leptin and thyroid hormones were determined in the blood serum of lambs during both periods. In the blood of fattening lambs significantly higher (P0.05) insulin concentrations (1.05 and 0.54 μU/mL), were determined, compared to suckling lambs. A significant strong positive correlation between serum leptin and insulin (r = 0.85, P0.05). The concentration of thyroid hormones did not significantly differ depending on the period of measurement. These changes indicate that the measurement concentrations of metabolic hormones and leptin in blood are very important in order to understand the changes of metabolism and nutrient supply in growing lambs., Cilj je istraživanja bio utvrditi koncentracije hormona koji reguliraju metaboličke aktivnosti i leptina u krvi janjadi u porastu. Istraživanje je provedeno s janjadi cigaja pasmine u sisajućem razdoblju i tovu tijekom zimske sezone hranidbe. Janjad je sisala i dobivala krmnu smjesu i sijeno lucerne po volji, dok je tijekom tova hranjena samo navedenom smjesom i sijenom lucerne po volji. Uzorkovanje krvi provedeno je 35. dana u sisajućem razdoblju i 75. dana u tovu. U krvnome serumu janjadi utvrđene su koncentracije inzulina, leptina i hormona štitaste žlijezde tijekom oba razdoblja. U krvi janjadi u tovu mutvrđena je značajno viša (P0.05), u odnosu na sisajuću janjadi. Također je u krvi janjadi u tovu utvrđena jaka pozitivna korelacija između koncentracija leptina i inzulina (r = 0,85; P0,05). Koncentracija hormona štitaste žlijezde nije značajnije odstupala ovisno o razdoblju mjerenja. Navedene promjene ukazuju da je mjerenje koncentracija hormona koji reguliraju metaboličke aktivnosti (insulin i hormoni štitaste žlijezde) i leptina u krvi vrlo važno, s ciljem razumijevanja mijene tvari i hranidbene opskrbe janjadi u porastu.