The proteins synthesized in vitro by rat brain, heart, and lung slices were compared by two-dimensional polyacrylamide gel electrophoresis. A protein, P71, which is related to one of the heat shock proteins induced in many cultured cells by hyperthermia and other harsh conditions, was the major protein synthesized by all slices from rats 3 weeks old and older. In vivo synthesis of P71 was not detected in brain, heart, or lung from these animals nor was there any detectable Coomassie brilliant blue-stained protein coinciding with P71 on the gels. P71 thus appears to be a minor protein species in normal unstressed brain, heart, and lung. While both heart and lung slices synthesized large quantities of P71 at all stages of postnatal development, brain slices synthesized little, if any, P71 until 3 weeks postnatal. There was a dramatic decrease in protein synthesis in all tissue slices during postnatal development. During this decrease, the relative abundance of newly synthesized P71 remained almost constant in heart and lung slices, but the relative abundance of P71 increased by approximately 50-fold in brain slices. The cells synthesizing P71 in brain slices were enriched in a microvascular fraction. The increase in P71 synthesis by these cells, occurring between the 3nd and 3rd postnatal week, coincides with the final maturation of brain capillaries and the blood-brain barrier.