LH-RH injection and infusion studies were performed in advanced puberty, delayed puberty and hypogonadotrophic hypogonadism. No differential diagnosis could be made between delayed puberty and hypogonadotrophic hypogonadism using LH-RH injection. In the LH-RH infusion studies evidence was obtained that stimulation of the pituitary during 4 h results in continuously rising LH levels in advanced puberty and in delayed puberty while in hypogonadotrophic hypogonadism the secretory capacity of the pituitary is gradually exhausted. This phenomenon can be used in the differential diagnosis between delayed puberty and hypogonadotrophic hypogonadism. Though the FSH data point in the same direction they are not useful in this connection as the overlap between the different categories was considerable.