A fly ash extract from a municipal incinerator, containing polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), was orally administered to rats on Days 10 to 17 of the pregnancy and during the first 10 days of the lactation period. PCDD and PCDF congeners retained in the fetuses and livers of the offspring had a 2,3,7,8-chlorine substitution pattern. The only non-2,3,7,8-substituted congener retained in the liver of the offspring and females was 2,3,4,6,7-pentachlorodibenzofuran (PnCDF). In the fetuses 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) had the highest retention (0.13% of total dose) and a decrease in retention was found with increasing number of chlorine atoms. In the livers of the offspring, the highest retention was found for 2,3,7,8-TCDD and the three 2,3,7,8-substituted hexachlorodibenzo-p-dioxins (HxCDDs) (5.26-8.12%). In the livers of the pregnant and lactating females penta- and hexachlorinated congeners had the highest retention (53.91-80.20%). For both groups the liver retention of the tetra- to octachlorinated congeners was similar, but the lactating females stored less PCDDs and PCDFs in their adipose tissue. A linear relationship was found between the retention of congeners in the livers of the females and offspring.