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34 Our recent experiments showed that gonadotropin(s) stimulated activin βA and follistatin expression through the cAMP-PKA pathway but suppressed βB via a cAMP-dependent but 36 PKA-independent pathway in cultured zebrafish follicle cells. Given that pituitary gonadotropins are the major hormones controlling the development and function of the ovary, the 38 differential expression of activin βA and βB as well as follistatin in response to gonadotropin(s) raises an interesting question about the temporal expression patterns of these molecules in vivo 40 during sexual maturation and ovulatory cycle. Three experiments were performed in the present study. In the first experiment using sexually immature zebrafish, we followed the expression of 42 activin βA, βB and follistatin at the whole ovary level during a 10-day period in which the ovary developed from the primary growth stage to the one with nearly full-grown follicles. Activin 44 βA expression was very low at the primary growth stage but significantly increased with the growth of the ovary, and its rise was accompanied by an increase in follistatin expression. In 46 contrast, the expression of activin βB could be easily detected in the ovary of all stages; however