The liver is a vital organ in the body responsible for a variety of functions that support metabolism, immunity, digestion, toxin removal, and vitamin storage, among others. Therefore, the aim of this study was to determine the most important histological changes in liver cells of laboratory-bred white rats after birth for the days (1, 3, 5, 7, 10). Additionally, it aimed to assess the activity of liver cells during the same period using the MTT assay. This study was conducted at the animal facility of the College of Veterinary Medicine, Tikrit University, Iraq. Eight rats were used in the experiment, divided into five females for breeding purposes and three males for mating. Subsequently, tissue sections of selected organs (liver parts) were prepared using hematoxylin and eosin staining. The results of these sections showed clear histological developments and changes in those cells, along with their high activity during the developmental process, consistent with the extent of development occurring during that period.