These studies investigated the maternal transfer of xenobiotic ligands in the estuarine mummichog (Fundulus heteroclitus). The common environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo[a]pyrene (B[a]P) were shown to accumulate in maturing oocytes of gravid F. heteroclitus. In vivo, time-course experiments with gravid female fish demonstrated time- and dose-dependent changes in oocytic accumulation of 3H-TCDD and 14C-B[a]P. In studies with 14C-B[a]P, both total radiochemical content and the composition of 14C-B[a]P metabolites in these extracts presented an accumulation profile that mirrored the profiles observed in sera from dosed fish reported previously. Furthermore, the accumulation of both 3H-TCDD and 14C-B[a]P was related to oocyte maturational state (i.e., size) and, correspondingly, to vitellogenin (VTG) uptake. Although VTG uptake is critical for proper oocyte growth and early-life-stage development, analysis of these data suggests an additional role for VTG as a vector of maternal transfer for both 3H-TCDD and 14C-B[a]P. This conclusion is also supported by an accompanying paper, presented in this volume, that examined the association of VTG with TCDD and B[a]P.