Objective To quantitatively study the first year development of key structures in the rat spleen. Methods Animals (6-7 per age group) were randomly sampled from a same cohort of normal male Sprague-Dawley rats at different phases of development -1 day and 1, 3, 6 and 12 months. The whole spleen was excised and tissue blocks were obtained for preparation of methacrylate embedded sections. The sections were stained with periodic acid-Schiff’s reagent and hematoxylin and the total volumes of key histological structures in the spleen were quantitatively studied using stereological methods. Results From the age of 1 day to 1 month, the splenic volume increased 7.7-fold; from 1 month to 3 months, it increased 3.0-fold; and from 3 to 12 months, it increased by 39% only. At 1 day of age the periarteriolar lymphoid sheath was formed in the rat spleen; at 1 month the marginal zone was formed; and at 3 months the splenic follicle was formed and blood was filled in the splenic sinus. At 3 to 12 months, the volume percentages of the white pulp and the red pulp in the spleen became stable, being 23%-27% and 67%-70%, respectively. Conclusion The rat spleen develops rapidly and reaches maturity by 3 months of age, with some continual growth but small changes afterwards.