The aim of this study was to compare the responses of pituitary gonadotropins and the free α-subunit (FAS), measured by sensitive immunofluorometric assays at 0, 30 and 60 min after 100 µg GnRH i.v. in a group of 32 normal prepubertal boys (NB) and a group of 17 adult males with complete hypogonadotropic hypogonadism (HH). At time 0 min, medians of the serum levels of LH, FSH and FAS were not different in both groups. At 30 min (29.81 and 8.56 pmol/l) and 60 min (24.10 and 7.74 pmol/l) as well as peak levels (29.76 and 8.56 pmol/l), ‘deltas’ (peak minus baseline; 25.89 and 0.34 pmol/l) and ‘fold’-increase (peak divided by baseline; 7.0-fold and 1.6-fold), the medians were significantly higher in NB than in HH, respectively. There were fewer overlapping values of FAS between the 2 groups and they occurred in only 4% of the total number of subjects when ‘fold’ was considered. The measurement of FAS by a very sensitive immunofluorometric assay, after GnRH, distinguishes responses of NB from those of adult men with HH, that is of functional from permanent HH.