2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds elicit multiple effects on the function of adipose tissue and adipogenic cell lines, including the suppression of adipocyte differentiation. We began to examine the mechanism by which TCDD inhibits differentiation of the established preadipocyte cell line 3T3-L1. Examination of the expression of several early marker genes of preadipocyte differentiation through Northern blot analysis and of differentiation-dependent mitosis showed that TCDD did not interfere with the earliest known responses of preadipocytes to inducers of differentiation. Analysis of mRNA for three isoforms of the CCAAT/enhancer binding protein (C/EBP) revealed that TCDD (5 nM) selectively inhibited the induction of C/EBP alpha mRNA but did not block the induction of C/EBP beta and C/EBP delta in response to differentiation inducers. The differentiation-dependent induction of PPAR gamma mRNA was also blocked by TCDD. Immunoblot analysis with specific antibodies to each C/EBP isoform demonstrated that the levels of C/EBP delta and C/EBP beta protein were rapidly induced (by day 1) and then abrogated by day 4 and 8, respectively, in solvent-treated (control) cells. In TCDD-treated cells, however, the levels of C/EBP beta and C/EBP delta protein persisted at these time points. In contrast, C/EBP alpha protein was markedly suppressed by TCDD in concordance with its level of RNA. Both translational products of C/EBP alpha, p30 and p42, were dose-dependently decreased by TCDD. Gel shift analysis of nuclear extract binding to an oligonucleotide containing a C/EBP DNA recognition sequence revealed no difference between extracts from control and TCDD-treated cells in the binding pattern at day 2 of differentiation. At days 4 and 8, the band corresponding to the C/EBP alpha/DNA complex (as determined with supershift assays) was dramatically decreased in the treated extracts in comparison to control extracts. In contrast, a band corresponding to a C/EBP beta/DNA complex was found to be enriched in the treated samples. These data indicate that suppression of differentiation in the 3T3-L1 preadipocyte cell line by TCDD occurs at a short but defined period, during the differentiation program, and involves altered regulation of C/EBP, including the inhibition of C/EBP alpha.