CHO cells were heated at 43 degrees C or 45 degrees C for various durations up to 300 min. Survival values varied from 5 x 10(-1) to 10(-7). Unheated, non-tolerant control cells were compared with cells made thermotolerant (TT) by incubating at 37 degrees C for 6 or 12 h after treatment with either sodium arsenite (100 microM-As) or 45.5 degrees C for 10 min, respectively. Groups also were included in which heat-induced TT cells were heated at 43 degrees C for 5 h immediately before they were challenged at 45 degrees C; in these groups, cycloheximide was sometimes added to inhibit protein synthesis before and/or during heating at 43 degrees C. Morphological alterations were quantified immediately and at various times after heating by using phase-contrast microscopy to determine the percentage of cells that were severely blebbed and rounded. About 800 cells were analysed per datum point. When effects of heat on thermotolerant cells were compared with effects of heat on non-tolerant cells, heat-induced thermotolerance (HTT) was observed by an increase in survival, and by a reduction in the percentage of cells with morphological alteration observed immediately after the challenging heat. After the As treatment, very little thermotolerance was observed for morphological alterations immediately after the challenging heat, although thermotolerance was observed for survival. However, as the cells were incubated for 12 or 24 h at 37 degrees C after the challenging heat treatment, recovery from morphological alterations was observed in the As-TT cells. Possible mechanisms for the difference between HTT and As-TT are discussed.