This paper is a historic review of two studies of the treatment of cryptorchidism. One was concluded 20 years ago with native LHRH, while the other took place 14 years ago and involved one of the potent LHRH analogues. The first study was initiated at the end of 1971-at a time when the different modes of action of the decapeptide were by far not evaluated to the full extent they are now-and included ten prepubertal boys with undescended testes aged five to ten years. The treatment consisted of intramuscular application of 20 micrograms LHRH once daily for three weeks. Additionally, three boys, five, seven and nine years of age, also presenting undescended testes underwent an LHRH test with intravenous bolus application of 200 micrograms. In all three subjects, a 2.5-fold increase in plasma testosterone was observed 20 min after the bolus. The treatment group showed neither a rise in LH nor in testosterone at the end of therapy. Testicular descent was achieved in six out of six boys with bilateral and in 2/4 with unilateral undescended testes. These good results might be based on the fact that six boys had retractile rather than actually undescended testes. In the second study started in 1978, 18 boys, three to 12 years of age with either unilateral or bilateral cryptorchidism, were treated with a synthetic LHRH analogue ("D-Leu 6, Des-Gly-10 LHRH ethylamide") applied intranasally. The peptide prepared in an aqueous solution was administered in the form of nose drops.(ABSTRACT TRUNCATED AT 250 WORDS)