Previous studies have shown that high dose 17β-estradiol (10-5 M) has a G2/M blocking effect in SW-13 human adrenal carcinoma cultures and strongly enhances apoptosis. To examine the differential effects of estrogen α and β-receptors in this system, we incubated SW-13 cells with specific α- and β-estrogen receptor agonists, PPT [4,4′,4′′-(propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol] and DPN [2,3-bis (4-hydroxyphenyl) propionitrile], respectively (each at 10-5 M). Flow cytometry was used to analyze the percentages of cells in various phases of the cell cycle [sub-G1 (apoptosis), G1, S, and G2/M] in each experimental condition. Exposure to 17 β-estradiol for 48 hours increased apoptosis more than 5-fold (from 3.6±0.5 to 20±2.2% of cells; p<0.01). The α-estrogen agonist PPT had a similar effect, increasing apoptosis to 22±1.7% (p<0.01), but the β-agonist DPN caused no change (3.6±0.5 vs. 3.9±0.8%). While estrogen and the α-estrogen agonist decrease apoptosis in this system, both of these compounds decreased the percentage of cells in G1 (from 59±1.4% for control to 34±2.3% for estrogen and 40±2.0% for PPT; p<0.01 for both agents relative to control); the β-agonist again had no effect. Estrogen was also found to block the cell cycle in G2/M, increasing it from 15±0.4 to 21±1.0% of cells (p<0.01), but neither the α- nor β-estrogen agonists had any effect at this point in the cell cycle, indicating that the influence of estrogen was not likely to be either α- or β-receptor mediated. There was no apparent effect of any of these agents on DNA synthesis, as indicated by unchanged percentages of cells in S phase. These studies suggest that induction of apoptosis by estrogen in SW-13 human adrenal cortical carcinoma cultures is mediated by the α-receptor, but the G2/M blocking effect of estrogen is not likely to be related to either α or β mechanisms.