Abstract. The aim of the present study was to estimate the genetic parameters and trends for fat and protein percentages of milk in Iranian Holsteins calving between 2001 and 2010 using the random regression test day model. Data set included 505 160 test day records that were collected by the Animal Breeding Centre of Iran. The Legendre polynomial functions of orders (5, 5) and (5, 6) were chosen to fit the additive genetic and permanent environmental effects of fat and protein percentages, respectively. Estimated heritabilities ranged from 0.053 to 0.232 and 0.111 to 0.259 for fat and protein percentages, respectively. The averages of estimated breeding values were −8.61 and −3.15 and annual genetic trends were −0.74 kg and −0.64 kg for fat and protein percentages, respectively. Negative genetic trends for fat and protein percentages are likely the result of major emphasis on milk yield in the breeding plan of Iranian Holsteins.