A report linking human polycystic ovary with increased exposure to environmental DDT (Heinrichs et al. 1971) prompted the present study comparing effects of PCB and DDT or their combination on reproduction in female rats under more realistic conditions with respect to level (75 and 150 ppm), route of administration (dietary contaminant), and period of exposure (8 and 36 weeks). Evaluation of estrous cycle length, mating frequency, number and size of litters; as well as plasma levels of DDT, PCB, progesterone (P), and 17 alpha=hydroxyprogesterone (17 alpha=OH-P), permitted comparison of short and long term reproductive changes from ingestion of two levels of DDT and/or PCB. PCB reduced plasma progesterone (p less than .01) while plasma 17alpha OH-P was unchanged by PCB or DDT. High DDT and PCB abolished reproduction. Histologically, distinct ovarian stromal changes accompanied 150 ppm of PCB, while increased numbers of more prominent follicular cysts were evident with 150 ppm of DDT. Although DDT and PCB generally reduced or abolished litter production, no treatment tested significantly altered litter size. Long term chronic ingestion of more realistic levels of technical DDT (85% p,p', 15% o,p'-DDT) in these studies did not lead to polycystic ovaries in adult rats comparable to those reported following i.v. administration of pure o,p'-DDT to immature rats. Plasma DDT levels above 800 ppb are clearly detrimental to reproduction, while levels below 500 ppb had lettle effect. Finally, we present the first evidence reported to our knowledge demonstrating that prolonged ingestion of PCB (150 ppm) markedly reduces reproduction (p less than .05) accompanied by significantly reduced progesterone in plasma (p less than .01) as well as by histologically characteristic ovarian stromal changes not seen with DDT alone.