The expression and cellular localization of alpha B-crystallin during rat renal development was studied by Northern blot analysis and by immunocytochemistry. Northern blotting of total RNA extracted from whole kidneys revealed that the messenger RNA for alpha B-crystallin rapidly increased after birth to reach adult levels by 20 days. At the same time, immunohistochemistry for alpha B-crystallin demonstrated that the prominent elongation of Henle's loop during the first 10 days of life was accompanied by increased alpha B-crystallin expression. Thus, the development of alpha B-crystallin expression is correlated temporally with the acquisition of tubule function in early post-natal life.