The opioid growth factor (OGF; [Met(5)]-enkephalin) and the OGF receptor (OGFr) form an endogenous and tonically active growth-regulating system that modulates cell proliferation by upregulating the cyclin-dependent kinase inhibitory pathway. Previous reports have documented that nucleocytoplasmic trafficking of the OGF-OGFr axis is dependent on nuclear localization signals. This study determined the specific transport factors required for the import of the OGF-OGFr complex from the cytoplasm to the nucleus using a probe of full-length OGFr fused to enhanced green fluorescent protein (eGFP) and knockdown of karyopherin alpha1, alpha2, alpha3, alpha4 or alpha6, karyopherin beta1 or Ran with small interfering RNA (siRNA). A human squamous cell carcinoma of the head and neck cell line (squamous cell carcinoma-1, SCC-1) that was downregulated for karyopherin beta1 or Ran did not have transport of OGFr-eGFP into the nucleus. Moreover, there was an increase of 44% in bromodeoxyuridine (BrdU)-labeled cells in cultures of SCC-1 that were transfected with siRNAs to karyopherin beta1 or Ran compared with cells transfected with scrambled siRNA. No alteration in distribution of OGFr-eGFP or BrdU labeling indexes was recorded in cultures treated with siRNAs to karyopherin alpha1, alpha2, alpha3, alpha4 or alpha6. These results indicate that the regulation of cell proliferation by the OGF-OGFr axis is dependent on nucleocytoplasmic transport by karyopherin beta1 as well as the gradient of RanGTP/RanGDP across the nuclear envelope, but is not reliant on adaptor molecules related to karyopherin alpha. Thus, the passage of the OGF-OGFr complex has controlled entry from the cytoplasm to the nucleus, and the timely and faithful translocation of this cargo across the nuclear envelope is critical to cell proliferation. These hierarchical levels of nuclear import provide multiple pathways for the subtle regulation of OGF-OGFr as it relates to the control of cell proliferative events.