The nuclear phenotypes and survival of the hemipteran, Triatoma infestans (Hemiptera, Reduviidae), were studied in specimens treated with copper sulfate and methylmercury. The objective was to determine whether changes in chromatin supraorganization and insect survival similar to those promoted by other stressing agents could also be induced by heavy metals. At the concentrations used, copper sulfate and methylmercury were toxic to the cells, mainly inducing nuclear degeneration in the Malpighian tubules and being lethal to a large part of the insect population. Although some individual resistance was found, especially in fasted specimens, heavy metals were found to be much more lethal than was, for instance, a thermal shock at 0 degrees C for 12 h. The nuclear phenotypes detected after heavy metal treatment were similar to those reported under other stressing conditions. However, the frequency at which nuclei exhibited aspects of heterochromatin unraveling was much higher than that found in fasted and thermal-shocked specimens, and was independent of the heavy metal type used. If this phenotype represents an attempt to improve opportunities for extensive cell and insect survival, it was not sufficiently effective. In 5th instar nymphs, the effect of CuSO4 on chromatin supraorganization was detected at early steps of spermatogenesis but not in the cells which were at late spermiogenesis when the metal was administered. This is probably due to changes in nuclear protein composition and to the tightly packed state of the DNA- protein complexes occurring at spermiogenesis, which may protect chromatin from damages. However, when CuSO4 was supplied to 4th instar nymphs, it slowed down the spermiogenesis process, possibly due to several factors including Cu2+ binding to DNA phosphates.743209218