Background and Aims : Activin was originally found as a stimulating factor of follicle stimulating hormone in pituitary cell cultures. To establish a simple culture system for studying the effects of activin on prolactin production, we examined the effects of activin in rat somatolactotroph derived GH3 cells cultured in a serum free condition. Methods : GH3 cells were cultured in Ham's F10 medium containing 2 mM L-glutamine and 0.1% (w/v) BSA with activin, follistatin (an activin binding protein), or a vehicle (control). Results : The cell viability was between 80 to 90% throughout the 96 hour incubation period in the three groups. Activin stimulated prolactin production in a time dependent (24-96 hours) and dose dependent (75-300 ng/ml) manner. A co-treatment with graded doses of follistatin and 100 ng/ml of activin suppressed the increase of prolactin production brought about by activin in a dose dependent manner, and the production decreased to the control level after the addition of 300 ng/ml of follistatin. Conclusions : The present study demonstrated the stimulatory effect of activin on prolactin production and the antagonism between activin and follistatin in GH3 cells in a serum free condition. This culture system is thus useful to study the effects of activin on prolactin production.