Hives. Weals on the skin of a 67 year old male patient caused by urticaria. Urticaria, also known as hives and nettle rash, leads to itchy raised red rashes (weals) on the skin. The weals are caused by the release of chemicals into the tissues, which make small blood vessels leak fluid into the skin. Each weal may last for a few minutes or several hours. Urticaria is most often caused by a non-allergic reaction to foods or drugs, although it may also be caused by heat, cold, chemicals, stress and allergies among many other things. Treatment is with antihistamines and avoidance of the cause, if known.