To determine differences in the patterns of expression of Drosophila small heat shock proteins (shsp) during normal development in the absence of stress, proteins obtained from head, thorax and gonads of young (0-12 h, 3 days), middle-aged (3-6 days) and 15- to 20-day-old adult flies were separated on SDS-PAGE gels and blotted with monoclonal antibodies against hsp23 and hsp26. hsp23 was found in the heads and gonads of young males and females. In contrast, the maximum expression of hsp26 was seen in gonads of young flies, and it was only lightly detected in the brain. The expression of both proteins decreased as flies aged. This age-related decrease was particularly striking for hsp23 in females. The immunoblot results obtained were complemented by immunostaining of thin parasagittal sections of whole fly bodies. Hsp23 was found to be expressed in the brain, thoracic ganglion, fat body and gonads of young (0-12 h) males and females. On the other hand, hsp26 was essentially detected in ovaries and testes of these young flies. The analysis of the tissue expression of both proteins demonstrate that each shsp has a distinct cellular localization. In the central nervous system, hsp23 and hsp26 were present in the neurocytes of the brain and the thoracic ganglion. In addition, hsp23 (but not hsp26) was also detected in the central neuropile of these two organs. In testis, hsp26 was localized in the cytoplasm of spermatocytes and, probably, in the spermatid bundles. In contrast, hsp23 was detected at the periphery of cells (membranes).(ABSTRACT TRUNCATED AT 250 WORDS)