Incubation of 4th instar larvae of Chironomus tentans at elevated temperatures leads in salivary and Malpighian chromosomes to the appearance of 4-5 new puffs. Previously present puffs, particularly Balbiani rings in salivary chromosomes, become drastically reduced. The reactions of region IV-5C and Balbiani ring 1 and 2 in salivary glands are quantitatively analyzed. Statistically significant heat shock effects are observed already after 5 min and reach a maximum between 30 and 60 min. The effective temperature range is small (between 33 to 40 degrees C) with an optimum at 37 degrees C. Above 40 degrees C, i.e., at overheat shock temperatures, heat shock reactions are suppressed. Larvae heat or overheat shocked for 1-7 h or 15-30 min, respectively, survive when returned to normal culturing temperatures. The recovery from heat shock of the puffing pattern occurs in two phases: a fast one (10-20 min) and a slow one (up to 5 h) sometimes separated by a period of backlash. Quenching of overheat shocked larvae does not result in a delayed heat shock reaction.