The POU-homeodomain transcription factor Pit-1 governs the pituitary cell-specific expression of Pit-1, growth hormone (GH), prolactin (PRL) and thyrotropin ß (TSHß) genes. Alternative splicing generates Pit-1ß, which contains a 26-amino acid ß-domain inserted at amino acid 48, in the middle of the Pit-1 transcription activation domain (TAD). Pit-1ß represses GH, PRL and TSHß promoters in a pituitary-specific manner, since Pit-1ß activates these same promoters in HeLa nonpituitary cells. Here we comprehensively analyze the role of ß-domain sequence, position and context, to elucidate the mechanism of ß-dependent repression. Repositioning the ß-motif to the Pit-1 amino terminus, hinge, linker and carboxyl terminus did not affect its ability to repress basal rat (r) PRL promoter activity in GH4 pituitary cells, but all lost the ability to repress Ras-induced rPRL promoter activity. To determine whether ß-domain repression is independent of Pit-1 protein and DNA binding sites, we generated Gal4-Pit-1TAD, Gal4-Pit-1ßTAD and Gal4-ß-domain fusions and demonstrated that the ß-motif is sufficient to actively repress VP16-mediated transcription of a heterologous promoter. Moreover, ß-domain point mutants had the same effect whether fused to Gal4 or within the context of intact Pit-1ß. Surprisingly, Gal4-ß repression lost HDAC sensitivity and pituitary specificity. Taken together, these results reveal that the ß-motif is a context-independent, modular, transferable and dominant repressor domain, yet the ß-domain repressor activity within Pit-1ß contains cell-type, promoter and Pit-1 protein context dependence.