The effect of inbreeding on survival after a short-term heat shock was tested for two age groups of the cactophilic fruit fly, Drosophila buzzatii, reared under nonstress conditions. Four inbreeding levels (F = 0, F = 0.25, F = 0.375, F = 0.5) were generated by outcrossing or full-sib mating. All flies were conditioned at 36.5 degrees C for 75 min prior to exposure to stress, to activate the synthesis of heat-shock proteins. These proteins are known to protect cells against stress damage. The younger group of flies were exposed to a thermal stress of 40.7 degrees C for 88 min, 103 min, or 118 min and the older flies to the same temperature only for 88 min or 103 min, as the survival of older flies after heat stress was much lower than that of the younger flies. Survival after heat shock declined with increased inbreeding in both age groups. For the younger flies, the slope of the regression line, F, on survival was lower at higher stress levels. For the older flies, inbreeding effects were similar at both stress levels. Mortality without stress also differed significantly among inbreeding groups, mainly because of a large difference between the F = 0.5 group and all others.