Dieldrin-induced increases in cytosolic cytochrome c and caspase-3 activity in N27 cells. In A, N27 cells (5 × 106 cells) were exposed to 100 or 300 μM dieldrin for 15–30 min. The mitochondria-free cytosolic fraction was collected as described in "Materials and Methods," and cytosolic cytochrome c was measured using ELISA cytochrome c assay. Data represent the mean ± SEM for three separate experiments performed in triplicate. Significance was determined at *p < 0.05 or **p < 0.01 compared with the vehicle-treated group at each time point. In B, N27 cells (2 × 106 cells/ml) were exposed to 30–300 μM dieldrin for 3 hr at 37°C, and caspase-3 activity was measured using the caspase-3 specific substrate Ac-DEVD-AMC, as described in the "Materials and Methods." The data are expressed as fluorescent unit (FU) per mg protein per hr of incubation. Each point represents mean ± SEM from two separate experiments in triplicate. Significance was determined at *p < 0.05 or **p < 0.01 compared with vehicle-treated cells.