SOCS3 is expressed by lymphoid cells, and can modulate the sensitivity of these cells to cytokine stimulation through inhibition of JAK/STAT signalling pathways. This study employed a mouse pro-B cell line expressing the human GH receptor (BaF3/hGH), to elucidate the signal transduction pathways used by GH to elicit SOCS3 expression. GH treatment of these cells caused a rapid, dose dependant increase in SOCS3 mRNA expression, which was independent of de novo protein synthesis. As expected, GH treatment increased JAK-dependant STAT5 tyrosine phosphorylation, which bound to the proximal STAT response element (pSRE) on the SOCS3 promoter. This process appeared to involve STAT5b, rather than STAT5a. In addition, GH activation of the SOCS3 promoter required a nearby AP1/CRE response element, which bound CREB, c-Fos and c-Jun. Moreover, inhibitors of p38 MAPK and JNK prevented GH-stimulation of SOCS3 mRNA expression in these cells, suggesting a role for these kinases in SOCS3 transcription. Importantly, GH stimulation increased binding of FOXO3a to the SOCS3 promoter at a site overlapping with the AP1/CRE response element, and over-expression of FOXO3a in these cells augmented SOCS3 promoter activation. In addition, we show a direct interaction between FOXO3a and STAT5 in these cells, which may provide a link between STAT5 and the AP1 transcription factors on the SOCS3 promoter. We conclude that regulation of SOCS3 expression by GH in a pro-B cell involves not only the pSRE, but also a transcriptionally active complex involving CREB/c-Fos/c-Jun and FOXO3a. This study has implications for cytokine regulation of SOCS gene expression in lymphoid cells.