Heat-shock protects against transient environmental conditions that mi
ght otherwise be lethal to the organism. Daphnia magna (2-day- and 7-d
ay-old) exposed for 4h to 34 degrees C showed an increased tolerance t
o a lethal malathion (0.1-0.2 ppm) challenge. Tolerance was measured u
sing two toxicity endpoints, survival and acetylcholinesterase (AChE)
activity. Survival among daphniids previously exposed to 34 degrees C
was greater than in daphniids previously exposed to 32 degrees C, 30 d
egrees C, and malathion, or in untreated (20 degrees C) animals. The 3
4 degrees C animals had a smaller decrease in AChE activity when compa
red to the other pretreatments. A similar increase in tolerance was ob
served among 2-day-old daphniids. Other endpoints were examined during
the various treatments. Elevated levels of heat-shock proteins (HSPs)
were present in daphniids exposed to 34 degrees C, but in no other tr
eatment. Aliesterase activity was reduced with heat-shock, but was not
significantly reduced in the malathion challenge, regardless of the p
retreatment. An apparently heat-inducible glutathione S-transferase (G
ST) was detected after the 34 degrees C heat-shock pretreatment, and t
his GST was present during the lethal malathion challenge.