Acetylcholine is a neurotransmitter that has been implicated in the pathophysiology of major depression. This is supported by the enhanced growth hormone (GH) release in response to pyridostigmine (PYD) challenge in depressed subjects relative to healthy comparison subjects. The aim of this study is to examine the specificity of the PYD/GH challenge in the diagnosis of depression. Pyridostigmine 120 mg orally, was administered to a total of 116 physically healthy subjects. Growth hormone responses were studied in 38 patients with (DSM-III-R) major depression, 13 subjects with panic disorder, 9 subjects with schizophrenia, 10 recently detoxified alcoholics, and a comparison group of 46 healthy volunteers. Mean delta GH (the difference between basal and maximal GH following PYD) was significantly greater than comparison subjects in patients with major depression. Responses observed in patients with schizophrenia and alcohol dependence syndrome did not differ from the comparison group. Those patients with panic disorder and a high Hamilton depression score had an enhanced delta GH. The sensitivity of the PYD/GH test was 63% for major depression. These results indicate that the PYD/GH test may help distinguish depression from schizophrenia, alcohol-dependence syndrome, or panic disorder with a low Hamilton depression score.