Objective: To determine the effects of Vitamin A excess on postnatal thymic development in albino rats. Study design: Experimental Place and Duration of Study: This study was conducted in the Department of Anatomy/ Histopathology, Shaikh Zayed Postgraduate Medical Institute, Lahore from September 2008 to September 2009. Materials and Methods: Study was conducted with 36 rat pups, of Sprague dawley variety. They were randomly separated into two equal groups A and B (n=18) on postnatal day 1. Vitamin A was used in the form of isotretinoin (13-cis retinoic acid). Group A was control group while group B was experimental group. Oral dose of isotretinoin, 2.5 mg/ kg body weight, was given to the pups on every mid-day of the week, in subsequent four weeks of postnatal period (days 4, 11, 18 & 25). On 30th postnatal day all pups were euthanized and weighed. Their thymuses were then dissected for morphological study. Results: Pups of group B showed reduction in mean body weight when compared with control group (P<0.01). The mean thymocyte population of experimental group was found to decrease as compared to the pups in the control group (P<0.01). The mean percentage of immature thymoblasts of experimental group B was also found to decrease (P<0.05). Conclusion: Postnatal development of thymus in albino rats can be interrupted with frequent use of Vitamin A in terms of lymphocytes depletion.