In CHO cells, growth hormone stimulation induces a rapid degradation of mature and precursor forms of its receptor, but does not affect Jak2 concentration. Confocal analysis of the receptor and of specific markers for subcellular localization shows that ligand exposure induced the disappearance of cell surface receptors, while some receptors seem to be sequestered in the endoplasmic reticulum (ER) and in the Golgi apparatus. Using a tagged version of Jak2 (HA-Jak2) and double immuno-fluorescence analysis with anti-HA and anti-Stat5 antibodies, we demonstrate that ligand stimulation induces Stat5 nuclear accumulation while Jak2 remains localized in the cytoplasm. Immunoblots of nuclear extracts confirm the Jak2 nuclear exclusion.