HEK-293T cells transiently transfected with ovine (o) GH receptor (GHR) and prolactin receptor (PRLR) constructs respectively tagged downstream with cyan or yellow fluorescent proteins were used to study ovine placental lactogen (oPL)-stimulated heterodimerization by fluorescence resonance energy transfer (FRET) microscopy. The oPL-stimulated transient heterodimerization of GHR and PRLR had a peak occurring 2.5-3 min after oPL application, whereas oGH or oPRL had no effect at all. The results indicate none or only little dimerization occurring before the hormonal stimulation. The effect of heterodimerization was studied by comparing activation of Janus kinase 2, signal transducer and activator of transcription (STAT)1, STAT3, STAT5, and MAPK in Chinese hamster ovary cells stably transfected with chimeric genes encoding receptors consisting of cytosolic and transmembrane parts of oGHR and oPRLR, extracellular domains of human granulocyte and macrophage colony-stimulating factor (hGM-CSF) receptor α or β, and cells transfected with the two forms (α or β) of PRLR and GHR. Functionality of those proteins was verified by hGM-CSF-induced phosphorylation of both intracellular PRLR and GHR domains and hGM-CSF-induced heterodimerization was documented by chimeric receptor coimmunoprecipitation. Homodimerization or heterodimerization of PRLRs and GHRs had no differential effect on activation of STAT5 and MAPK. However, heterodimerization resulted in a prolonged phosphorylation of STAT1 and in particular STAT3, suggesting that the heterodimerization of α-oGHR and β-oPRLR is able to transduce a signal, which is distinct from that occurring on homodimeric associations.