The Huntington disease gene encodes the protein huntingtin, which is widely expressed during embryonic development and in mature tissues. In order to elucidate the physiological function of huntingtin, which so far is unknown, we intend to study the effect of antisense downregulated huntingtin expression. We have found an inhibiting effect of a phosphorothioated oligodeoxynucleotide (PS-ODN) added to the culture medium of embryonic teratocarcinoma cells (NT2) and postmitotic neurons (NT2N neurons) differentiated from the NT2 cells. Specific inhibition of expression of endogenous huntingtin was achieved in NT2N neurons in the concentration range of 1-5 muM PS-ODN, whereas no inhibition was obtained in NT2 cells. We describe in detail the selection of the target sequence for the antisense oligo and the uptake, intracellular distribution, and stability of the antisense PS-ODN in the two cell types. Antisense down-regulation of huntingtin in this model of human neurons represents a suitable approach to study its normal function.