As glutamine did not reduce nNOS activity in either TGW-nu-I neuroblastoma or A673 neuroepithelioma, endogenous citrulline does not seem to contribute to the supply of nNOS substrate in these cells, whereas 25% of NOS-substrate in endothelial cells is supplied by the recycling from endogenous citrulline to arginine. This evidence concerns the gene NOS1 and neuroblastoma.