To study the effect of photoperiod on lung associated cellular immunity in diabetic rats, different photoperiod regimens were used. These were - Long photoperiod (LP; 20L:4D), Short photoperiod (SP; 4L:20D) and natural day length for control group. The cell mediated immunity was evaluated in terms of size of Bronchus Associated Lymphoid Tissue (BALT) and non-BALT nodules and percent stimulation ratio of lung isolated lymphocytes which was supported by peripheral lymphocyte count (percent). Short photoperiod increased bronchusassociated lymphoid tissue (BALT) and non-BALT nodule size, peripheral lymphocyte count and percent stimulation ratio of lymphocytes. LP decreased the above-mentioned immune parameters suggesting that photoperiod might be regulating lung-associated immune system (LAIS) via melatonin during diabetic condition.