Thirty adult Merino rams were fed a ration with a 16% protein and 75% TDN content. Live body mass increased by 51,4% and testes volume by 111,7% in 210 days. Testicular growth responded rapidly to supplementation and testes volume increased by 86,5% in only 60 days. In another experiment diets of four groups of 15 Merino rams each, aged 14 months, were supplemented with lucerne hay, sunflower oil cake meal, fish meal and urea respectively, Testicular volume increased from 79,3to 104,0% in the four groups in a7-weekperiod, Differences were, however, non-significant (P > 0,05).