The molecular mechanisms underlying the effect of thyroid hormone (T₃) on neurite outgrowth are unknown. We recently identified the small GC-box binding protein BTEB (basic transcription element-binding protein) as a T₃-regulated gene in the developing rat brain. BTEB mRNAs are rapidly (by 1 h) up-regulated by T₃ in primary rat embryonic neuronal cultures. Antisense oligodeoxynucleotides (ODNs), added to the cultures, reduced by 60% the level of BTEB mRNA. Addition of BTEB antisense ODNs to the cultures, before the onset of neurite polarity, had no effect on neurite elaboration but significantly decreased, in a dose-dependent manner, the effect of T₃ on neurite branching. We then examined the effects of antisense ODNs on a thyroid hormone target neuronal population, i.e. the acetylcholinesterase-positive neurons after the onset of neurite polarity. Exposure to BTEB antisense ODNs completely abolished the effects of T₃ on neurite branching and on the elaboration of neuritic filopodia-like structures in acetylcholinesterase cells. By contrast, antisense ODNs did not alter the effect of T₃ on neurite length. Our results show that titration of BTEB levels by T₃ regulates the degree of neurite branching and that the T₃-induced neurite elongation and the T₃-induced neurite branching are regulated by distinct mechanisms.