Investigations were carried out on 40 rams aged 50 days (n = 8), Kamieniecka (K), Pomorska (P) and Polish Blackheaded Mutton (PBM) breeds and their crossbreeds (K x PBM, P x PBM). Microscopic evaluation of the liver, kidneys, spleen and heart muscle in the rams as well as ultrastructural analyses of their liver and semitendinous muscle showed that retrogressive lesions, circulation disturbances, inflammation and progressive changes occurred respectively: frequently, occasionally, rarely. Internal organs, particularly liver and kidneys, of crossbred rams (K x PBM and P x PBM) were almost two times more affected with morphological lesions than purebred lambs (K and P). However, in the semitendinous muscle these differences were more vivid in the ultrastructural analysis than in the histopathological or macroscopic observations. Results suggest that breed growth-rate differences have effects on the pathomorphological pattern of the liver and kidneys in lambs. On the bases of this evaluation, it can also be emphasised that young PBM rams are less susceptible to morphological lesions than the K and P breeds.