Diabetes Mellitus (DM) is a group of metabolic disorders with relative or absolute insufficiency of insulin secretion and/or concomitant resistance to the metabolic action of insulin on target tissues. Aim: to evaluate the effect of STZ-induced-DM on the dental pulp of albino rats histologically and ultra-structurally, and its effect on pulpal T&B lymphocytes immunohistochemically. Methods: Thirty male albino rats, were used in this study, divided into 2 groups: Group A (Control Group):10 rats received 2ml of citrate buffer (IP), was equally sub-divided into 2 sub-groups A1 & A2, according to the date of specimen collection. Group B (Experimental Group): 20 rats received an IP injection of STZ, equally sub-divided into 2 sub-groups; Sub-group B1 and B2 where the animals were sacrificed 2 and 4 weeks after establishment of DM respectively. permanent molars of each rat were split for pulp detachment, processed for histological, immuno-histochemical and ultra-structural examination. Results: H&E-stained sections of the pulp tissue of 2-weeks-diabetic rats showed degenerative changes. Ultra -structural examination showed degenerative changes affecting pulp elements. Immuno-histochemical examination of the pulp showed less T&B lymphocytes. Conclusion: DM led to degenerative changes of the pulp elements namely, cells, fibers, nerves, and vessels. DM also affected T&B lymphocytes.