AbstractThe effect of enhancement of cholinergic tone by pyridostigmine on the growth hormone (GH) response to thyrotropin‐releasing hormone (TRH) or glucose‐induced acute hyperglycaemia was tested in six adult unanaesthetized beagle dogs. Both TRH (5μg/ kg iv) and glucose (2 g/kg orally) did not significantly alter baseline GH levels but reduced the GH response to GH‐releasing hormone (GHRH) (2 μg/kg iv), although this effect was more clear‐cut with TRH than with glucose. Pretreatment with pyridostigmine (2 mg/kg orally) counteracted the inhibitory effect of hyperglycaemia on the GHRH‐induced GH release, but had no effect on the inhibition induced by TRH. In summary, these results indicate that: 1) acute hyperglycaemia and TRH play an inhibitory role on GHRH‐stimulated GH secretion in dogs; 2) the inhibitory effect of acute hyperglycaemia is mediated via hypothalamic cholinergic neurotransmission, whereas other neurotransmitter pathways would be. involved in the