In 11 untreated acromegalic patients the plasma GH levels were determined after the acute administration of bromocriptine, haloperidol, pimozide (only in 8 patients) and of placebo. A 50% or more suppression of the basal GH levels was arbitrarily defined as a positive response to bromocriptine. Five patients displayed a negative response to bromocriptine. Of these, 4 responded to both antidopaminergic drugs. We conclude that the acute administration of antidopaminergic drugs reduces the GH secretion in some bromocriptine-insensitive acromegalic patients. Therapeutical implications will require further studies.