Several tissues from the 3 oilseed Brassica species were analyzed for polypeptides synthesized in vivo and in vitro under control (23°) and heat-shock conditions. The various tissues have characteristically unique patterns of polypeptide synthesis, the general pattern being maintained following heat shock. In addition, all tissues exhibit synthesis of a set of qual. similar heat shock protein (HSP) that differ, however, in relative intensity among tissues. Synthesis of control polypeptides as well as high mol.-weight HSP is similar in hypocotyls of the 3 genomically interrelated species B. napus, B. campestris, and B. juncea; however, the 3 species exhibit variation in vivo and in vitro synthesized polypeptides in the 18,000-27,000 mol.-weight region, with several prominent differences in the number and distribution of the major class of low mol.-weight (20,000) HSP. Several of the HSP appear to be unique to B. juncea and one is unique to B. napus. Immunochem. studies of in vitro synthesized Brassica polypeptides using antisera to maize HSP of mol. weight 18,000 substantiate the observed differences in synthesis of various members of the 20,000 mol.-weight HSP family among the 3 species, and indicate that the Brassica low mol.-weight HSP are immunol. similar to the small HSPs of other plants including maize, soybean, pea, and Arabidopsis.