To evaluate the somatotropic function of the pituitary, the measurement of the spontaneous nocturnal secretion of GH is a rather suitable method. Whereas the provocation tests check the capacity of the gland after intensive stimulation, spontaneous secretion reflects the behaviour of the hypothalamohypophyseal system under everyday conditions. The investigation of 65 children with pituitary dwarfism showed in all cases a strong diminution of the GH secretion. Overlapping with the control group was hardly seen. The maxima measured during sleep were identical with those reached in the provocation tests. The subdivision in complete and partial GH deficiencies is more precise with measuring the spontaneous secretion than with provocation tests. Nocturnal spontaneous GH secretion was determined in 128 patients with constitutional delay of growth and adolescence. Also in these children provocation tests were performed simultaneously. Spontaneous GH secretion was found significantly diminished in all stages of puberty, compared to the controls (p less than or equal to 0.01). By contrast, the provocation tests showed no significant differences from controls. According to these results, the retarded growth of the patients is due to a relative GH deficiency. As is evident from the normal results of the provocation tests, no organic insufficiency of the pituitary is demonstrable. Rather a cybernetic disorder is responsible for the reduced hormone secretion.