The effects of tendon vibration on bioassayable growth hormone (BGH) secretion from the pituitary gland were investigated in anesthetized adult male rats. The tendons from predominantly fast ankle extensor muscles (gastrocnemius and plantaris) or a predominantly slow ankle extensor (soleus) were vibrated using a paradigm that selectively activates Group Ia afferent fibers from muscle spindles. The lower hindlimb was secured with the muscles near physiological length and the tendons were vibrated for 15 min at 150 Hz and a displacement of 1 mm. Control rats were prepared similarly, but the tendons were not vibrated. Compared to control, vibration of the tendons of the fast ankle extensors markedly increased (160%), whereas vibration of the slow soleus decreased (68%), BGH secretion. Complete denervation of the hindlimb had no independent effects on the normal resting levels of BGH, but prevented the effects of tendon vibration on BGH secretion. The results are consistent with previous findings showing modulation of BGH release in response to in vivo activation or in situ electrical stimulation of muscle afferents. These data provide evidence that this previously described muscle afferent-pituitary axis is neurally mediated via Group Ia afferents from peripheral skeletal muscle. Furthermore, these data show that activation of this Group Ia afferent pathway from fast muscles enhances, whereas the same sensory afferent input from a slow muscle depresses, BGH release.