Annexins V and VI, two Ca(2+)-dependent phospholipid- and membrane-binding proteins, were immunochemically measured in a number of rat organs and tissues during post-natal development. Annexin V proved much more abundant than annexin VI irrespective of the organ and the post-natal period considered. In the brain, annexin V accumulated at a high rate from the end of the first post-natal week onward, whereas annexin VI was expressed in extremely low amounts irrespective of the period investigated. In contrast, the levels of both annexins in the heart were nearly constant, in the post-natal period investigated. In skeletal muscles, annexin V and VI levels were high around post-natal day 1 and decreased thereafter. A similar pattern was observed for annexin V in liver, whereas the amounts of annexin VI in this organ were at the limits of detectability. In the lung, annexin V accumulated almost linearly from birth to adulthood, whereas annexin VI was relatively high at birth, decreased to low levels by the end of the first post-natal week, and re-accumulated thereafter. Among the organs examined, the lung and heart proved the richest sources of annexins V and VI. Annexin V appears to be a useful marker of and to be implicated in brain, lung and skeletal muscle maturation.