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High mobility group AT-hook 1 (HMGA1) is able to regulate gene expression and function as a tumour suppressor. The spatiotemporal expression pattern of HMGA1 was investigated in this study. Immature female rats (22–23 days old) were treated with 10 IU, s.c., pregnant mare's serum gonadotrophin to stimulate follicular development, followed 48 h later by injection with 5 IU, s.c., human chorionic gonadotrophin (hCG). Whole ovaries or granulosa cells were collected at various times after hCG administration (n = 3 per time point). Real-time polymerase chain reaction and western blot analysis revealed that HMGA1 was highly stimulated in the ovary by 4–12 h after hCG treatment. In situ hybridisation analysis demonstrated that Hmga1 mRNA expression was induced in granulosa cells between 8 and 12 h after hCG treatment. There was negligible Hmga1 mRNA signal observed in newly forming corpora lutea. In addition, the data indicated that both the protein kinase (PK) A and PKC pathways regulated Hmga1 expression in rat granulosa cells. In rat granulosa cell cultures, upregulation of Hmga1 was dependent on new protein synthesis because Hmga1 was inhibited by cycloheximide. Furthermore, Hmga1 mRNA expression in rat granulosa cell cultures was inhibited by AG1478, whereas NS398 and RU486 had no effect, suggesting that Hmga1 expression was regulated, in part, by the epidermal growth factor pathway. In summary, the findings of this study suggest that induction of Hmga1 may be important for theca and granulosa cell differentiation into luteal cells. Transcription factors are essential mediators of ovulation and luteinisation. The present study demonstrates that there are significantly increase in levels of the transcription factor high mobility group AT-hook 1 (HMGA1) after administration of human chorionic gonadotropin to rats. Induction of Hmga1 mRNA is mediated through both the protein kinase (PK) A and PKC signalling pathways by activation of the epidermal growth factor receptor and requires the synthesis of a new protein(s). Temporal stimulation of HMGA1 may be important for theca and granulosa cell differentiation into luteal cells. [ABSTRACT FROM AUTHOR]