Preliminary data from the National Institute of Mental Health-Clinical Research Branch Collaborative Program on the Psychobiology of Depression dealing with the human growth hormone (hGH) response to the Insulin Tolerance Test (ITT) during the pre-treatment (drug-free) period of the study are presented in this paper. Data are reported for 54 unipolar depressed, 21 bipolar depressed, and 40 normal control subjects, who represent approximately 50% of the final subject sample to be studied. In this population the unipolar depressed subjects showed a significantly greater resistance to insulin-induced hypoglycaemia than bipolar and control subjects. After applying the inclusion/exclusion criteria necessary to interpret hGH responses accurately, the data from only 54 subjects were acceptable. Mean peak hGH concentrations were not significantly different among the three groups. There was, however, a significant difference in the distributions of the hGH peak response, with the bipolar depressed population demonstrating greater variability in response than unipolar and control populations. These findings are discussed as they relate to previous reports and theoretical considerations.