Lithium impairs the appearance of the characteristic morphology of brown adipocytes and downregulates the expression of marker genes of brown adipocyte differentiation. These effects are dose‐dependent and are more pronounced when exposure of preadipocytes to lithium is initiated at early stages of differentiation. Although lithium reduces the expression of genes common to both white and brown adipocytes [fatty acid binding protein aP2 (aP2/FABP) or peroxisome proliferating activated receptor γ], genes expressed differentially in brown adipocytes, i.e., uncoupling protein 1, PPAR gamma coactivator‐1α, and peroxisome proliferating activated receptor α, are particularly sensitive to lithium treatment‐dependent downregulation. Brown adipocytes appear as preferential targets of the inhibitory action of lithium on adipocyte differentiation.