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diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) are persistent , lipophilic chemicals with different sources, pathways, and patterns of human exposure. Mixtures of PBDEs are present as noncovalently bound flame-retardant additives to synthetic fabrics, foams, or plastics in a variety of consumer products. Two mixtures, the penta-and octa-BDEs, are no longer used. Deca-BDE, the major PBDE with total production approaching that of PCBs at their peak, remains in use but is banned in Sweden, Maine, and Washington State. PBDE levels, where examined, have been increasing in humans and wildlife worldwide, doubling every 2–5 years (Hites 2004; Noren and Meironyte 2000; Sjödin et al. 2003a). PCBs have been banned for 30 years, but are present as dielectrics in telephone pole–mounted transformers common to urban areas. Exposure pathways for PCBs, as with most persistent organic pollutants, begin in the outdoor environment. Exposure pathways for PBDEs begin with indoor environments Little is known regarding the rates of loss (depuration) of PBDEs and PCBs from mothers during lactation. It is of interest to examine depuration of PBDEs and PCBs in breast-feeding mothers to determine whether depura-tion patterns reflect differences in sources, pathways, and patterns of human exposures. In a review of published data on depuration of persistent chemicals via breast milk, LaKind et al. (2001) indicated that most studies were not specifically designed to measure depura-tion rates, and were limited by combinations of small sample size (n = 1–3), few intra-individual measurements, interindividual measurements , or pooled samples. Preliminary data from an ongoing study suggest low depu-ration rates for PBDEs and PCBs (Sjödin et al. 2005). Our studies, initiated in March 2003, were designed to assess depuration patterns for PBDEs and PCBs using serial samples of breast milk from lactating mothers. Serial and precision studies. We examined depuration rates of 12 PBDE congeners and 80 PCB congeners by measuring concentrations in serial samples collected over extended periods from two groups (n = 9 for each)— short-term (ST) and long-term (LT)—each composed of volunteers from a cross-sectional study of California primiparae (CS CA) (n = 82) (Table 1). ST participants collected hand-expressed samples at fixed intervals (4, 6, 8, 12, 16, 20, and 24 weeks after infant birth) and a pumped sample at 6 weeks. Some ST mothers missed collections, and one ST mother collected 3 additional monthly samples , for a total of 10 samples over 38 weeks. LT participants collected two samples separated …