ABSTRACTIn this paper effects of major factors of genetic and paragenetic nature on milk yield ofexisting sheep populations are investigated. Mathematical procedures for evaluation of geneticand phenotypic parameters and anticipation of genetic values, today, are mainly based ondifferent linear and non-linear concepts, therefore it is necessary to accept the reality that there isno absolute accuracy.Results of the investigation have shown that there are positive and negative deviations inmonitored traits depending on the population, season, order of lactation, type of birth and littersize. Considering that, sheep milk has decisive role in production of cheese. Results obtained inthis research can be used as indicator for more efficient projection of improvement programmesin order to increase the production of milk and cheese.