The pattern of protein synthesis in roots of 3-day-old maize seedlings (Zea mays L.) is rapidly and dramatically altered when the incubation temperature is raised from 25 to 40 degrees C. One-dimensional sodium dodecyl sulfate gels reveal that although synthesis of the proteins observed at 25 degrees C continues at 40 degrees C, a new set of ;heat shock proteins' (hsp) is induced within 20 minutes of the temperature transition. The hsp have molecular weights of 87, 85, 79, 78, 77, 72, 70, 27, 22, and 18 kilodaltons. The 10 hsp are visible on autoradiograms but not on stained gels, suggesting that the proteins do not accumulate to any great extent.The induction of the hsp is transitory. With prolonged high temperature treatment, the synthesis of hsp continues for 4 hours in excised roots and for 8 hours in the roots of intact seedlings before declining sharply. Coincident to the decline in synthesis of the 10 hsp is the gradual increase in intensity of three new polypeptides having molecular weights of 62, 49.5, and 19 kilodaltons. These proteins begin to appear about the time that synthesis of the other 10 hsp becomes maximal.Shifting the temperature back to 25 degrees C also causes a decline in synthesis of hsp, but this decline occurs more rapidly than that seen during prolonged heat shock. A decrease in hsp synthesis becomes apparent 2 hours after the roots are returned to 25 degrees C.Shifting the temperature from 25 to 45 degrees C results in a pattern of protein synthesis different from that observed after a shift to 40 degrees C. Normal protein synthesis continues, except four proteins, which are produced in small amounts at lower temperatures, show greatly enhanced synthesis at 45 degrees C. These proteins have apparent molecular weights of 83, 81, 68, and 65 kilodaltons. Also, the 10 hsp listed above are not synthesized. It is suggested that at least two distinct high-temperature responses are present in maize, which may reflect the metabolic changes generated at different elevated temperatures.