SummaryThe spleen weights increase with age inRousettus aegyptiacusfor the first 6 months. The size of the adrenal cortex estimated by adrenal weight behaves in a similar way as the spleen weights. The spleen weights decline as the bats mature while the adrenal weights continue to increase with age.Both the spleen and adrenal weights are variable in pregnant bats. The pregnant animals have heavy adrenals and small spleens on average. Since the pregnant bats are heavy, their relative spleen and adrenal weights are different from the absolute weights, but more so in the latter than in the former.The spleen inRousettusresponds to stressful environments differently from the way it does in rodents under similar situations.