A single dose of pyridoxine(300 mg iv)produced significant rises in peak levels of immunoreactive growth hormone(GH)and significant decrease of plasma prolactin(PRL) in 8 hospitalized healthy subjects. Serum glucose, luteinizing hormone(LH), follicle stimulating hormone(FSH) and thyrotropin(TSH) were not altered significantly. In addition, in 5 acromegalic patients who were studied with both L-Dopa and pyridoxine, inhibition of GH secretion followed either agent in a similar pattern. These data suggest a hypothalamic dopaminergic effect of pyridoxine.