A sample of infants was examined at conceptual age 67 weeks (6-month testing, N=230), and again at conceptual age 92 weeks (12-month testing, N=216) using the Fagan Test of Infant Intelligence (FTII). Analysis of the results revealed a dose-dependent relationship between total umbilical cord-blood PCB levels and poorer FTII performance at both ages. A similar relationship was observed using a subset of the persistent and heavily chlorinated PCB congeners associated with Lake Ontario fish consumption (septa-, octa-, and nonachlorinated biphenyls) in children tested at 12-months but not at 6-months. These data replicate previous research (Jacobson et al., 1985) which demonstrated a dose-dependent relationship between prenatal PCB exposure and FTII performance in infants of Lake Michigan fisheaters. Analyses of FTII scores with cord-blood DDE and maternal hair methylmercury (MeHg) revealed no significant associations between FTII performance and either of these toxicants. The latter results replicate those of Myers et al., (1995) who found no effect of MeHg on FTII performance in Seychellois infants.