Activation of the aryl-hydrocarbon receptor (AhR) by pretreatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) blocks hormone (IDM/BRL)-induced adipocyte differentiation of C3H10T1/2 cells in proportion to the suppression of the elevation of the key mediator, peroxisome proliferator-activated receptor (PPARgamma1). Inhibition of MEK-induced ERK phosphorylation had no effect on adipogenesis but prevented this TCDD suppression. Initiation of MEK inhibition up to 6 h after IDM/BRL stimulation in combination with serum addition completely reversed the TCDD-mediated suppression but declined to ineffectiveness when delayed to 24 h after stimulation. This period occurs well after the decline of serum-induced ERK activation, at a time when ERK phosphorylation is low, and prior to the onset of IDM/BRL-stimulated PPARgamma1 expression. This temporal separation of ERK activation from the affected PPARgamma1 expression suggests that ERK does not act directly on either PPARgamma1 transcription or receptor function. Thus, ERK activation and TCDD/AhR stimulation work synergistically to inhibit adipocyte differentiation. Nonrenewal of serum at the time of IDM/BRL addition removed most of the ERK activation and also the TCDD-mediated suppressions of PPARgamma1 expression and adipocyte differentiation. Transfection of a vector expressing constitutively active MEK1 generated a constant, high level of phosphorylated ERK comparable to the peak serum-induced level and fully restored TCDD suppression without a TCDD-mediated effect on ERK phosphorylation. We conclude that low levels of activated MEK and ERK cooperate with AhR-induced factor(s) to generate a suppressor that prevents PPARgamma1 transcription and then differentiation.