The androgen receptor (AR) is a transcription factor that mediates androgen action. We have used the green fluorescent protein (GFP) technique to investigate dynamics of nuclear trafficking of human AR in living cells. In the absence of ligand, the GFP-AR fusion protein is distributed between cytoplasm and nuclei. Androgen exposure leads to a rapid and complete import of GFP-AR to nuclei of CV-1 cells (>=90% nuclear in 30 minutes), whereas a pure antiandrogen, Casodex, elicits a slower (<40% nuclear in 30 minutes) and incomplete transfer. Unliganded ARs with mutations in the basic amino acids of the bipartite nuclear localization signal (NLS) within the second zinc finger and the hinge region are predominantly cytoplasmic and their androgen-dependent nuclear import is severely compromised ((3/4)20% nuclear in 30 minutes). Interestingly, substitutions of the Leu residues flanking the bipartite NLS lead to inefficient nuclear transfer in response to androgen ((3/4)20% nuclear in 30 minutes). The ligand-binding domain of AR, which represses bipartite NLS activity, contains an agonist-specific NLS. The small nuclear RING finger protein SNURF, which interacts with AR through a region overlapping with the bipartite NLS, facilitates AR import to nuclei and retards its export on hormone withdrawal. More AR is associated with the nuclear matrix in the presence than absence of coexpressed SNURF. We suggest that the SNURF-mediated tethering of AR in nuclei represents a novel mechanism for activating steroid receptor functions.