1. Rates of gain and loss of acclimatization to temperature of males from two inbred lines and the hybrids between them were measured by recording their survival times in dry air at a lethal temperature (34°C).
2. All hybrid males lost acclimatization to temperature more quickly than did inbred males. B/K males gained acclimatization to temperature more quickly than any other group, but the K inbred males gained acclimatization more quickly than did either the K/B males or the B males. In all cases acclimatization is gained more quickly than it is lost.
3. The extent of acclimatization to temperature, as measured by the difference in survival times of 15°C. and 25°C. acclimatized flies in a range of lethal temperatures, was not found to be different in inbreds and hybrids.
4. The results suggest that hybrids can produce the enzymes necessary for acclimatization to temperature more rapidly than inbreds and confirms the hypothesis that hybrids are biochemically more versatile than inbreds.
5. The difference between the rates of gain and loss of acclimatization to temperature suggests that the processes involved in the enzyme changes are temperature dependent.
6. The absence of a difference in the extent of acclimatization to temperature indicates that both inbred and hybrid D. subobscura are capable of producing those enzymes necessary for temperature acclimatization.
7. The high values of the temperature coefficients for heat death indicate that this process involves protein (enzyme) denaturation.
8. An ageing effect was observed in inbred flies.