Activins are dimeric proteins of the transforming growth factor beta superfamily, which exhibit multiple functions in gonadal and extragonadal tissues. Expression of activin A, composed of two betaA subunits, has been shown in the thyroid, whereas there has been no study regarding activin B (betaBbetaB) in this gland. In other tissues, such as the gonads, pancreas, and adrenal cortex, expression of both activin betaA and activin betaB has been described. In this study, we detected activin betaB mRNA and protein expression using reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry in rat experimental goiter and in human thyroid, including multinodular goiter, follicular adenoma, papillary carcinoma, and follicular carcinoma. Activin betaA mRNA and protein expression was also investigated in rat and human thyroid tissue. The expression of both activin betaB and activin betaA was highest in rat methimazole-induced goiter and in human follicular adenoma, and papillary and follicular carcinomas when compared with multinodular goiter and normal thyroid tissue. The increased expression of activin betaB as well as activin betaA, observed in this study, suggests that activin B and activin A may be involved in the proliferative and neoplastic processes of the thyroid.