Fifth instar larvae of the tobacco hornworm, Manduca sexta, tolerate 1-h exposures to temperatures as high as 42 degrees C. Above 42 degrees C, survival declines rapidly to 18% at 44 degrees C and 0% at 48 degrees C. As in other insects, the heat-shock response of Manduca sexta involves the induction of synthesis of heat-shock proteins very similar in size to the Drosophila heat-shock proteins (84, 73, 71, 27, 25, 23, and 22 kd). In the epidermis, heat-shock protein synthesis peaks at 42 degrees C, correlating with the heat sensitivity of both the tissue itself and the intact larva. Some heat-shock proteins have different isoelectric forms depending on tissue. Also, the heat-shock proteins are synthesized over a wider range of temperatures in the imaginal discs and the fat body as compared to the epidermis. In contrast to dipteran insects, Manduca sexta does not exhibit a strong repression of non-heat-shock protein synthesis under tolerable conditions.