The minimal structural requirements for gonadotrophin releasing activity were studied with fragments of a highly active analogue of luteinizing hormone releasing hormone (LH-RH), [D-Ser(But)6]LH-RH(1-9)nonapeptide-ethylamide (Hoe 766). All fragments are related to the C-terminal structure of LH-RH and have increased enzyme stability. Ovulation in phenobarbitone-blocked rats was induced with a median effective dose/rat, of 1.9 microgram of the (3-9)-heptapeptide, Trp-Ser-Tyr-D-Ser(But)-Leu-Arg-Pro-ethylamide and 6.8, 18.0 and 38.3 microgram for the (4-9), (5-9) and (6-9) fragments respectively. The (3-9)- heptapeptide and (4-9)-hexapeptide induced release of LH and FSH in phenobarbitone-blocked rats with a ratio similar to that of LH-RH. Degradation of LH-RH by enzyme preparations of liver, kidney and hypothalamic or anterior pituitary tissue was not modified by addition of the (3-9)-heptapeptide fragment. The organ distribution of the 125I-labelled (3-9)-heptapeptide fragments was similar to LH-RH, but not to Hoe 766. The peptide accumulated in liver and kidney, but was eliminated from the anterior pituitary gland 15 min after i.v. injection, whereas Hoe 766 showed progressive accumulation in the pituitary gland (tissue:plasma ratio = 6.6 after 60 min). In contrast to C-terminal fragments of LH-RH, the corresponding fragments of nonapeptide analogues retained significant biological activity, and the minimal structural requirements for LH release may be related to the C-terminal sequence of LH-RH.