Contents: The effect of exogenous administration of lamprey GnRH-III (IGnRH-III) on gonadotropin secretion was evaluated in pigs. Six crossbred barrows (82.4 ± 3.5 kg body weight) were assigned randomly to a replicated 3 × 3 Latin square design to evaluate the effect of 0.1, 1.0 or 10.0 μg/kg body weight of exogenous IGnRH-III on LH and FSH secretion. To facilitate blood collection and infusion of IGnRH-III, barrows were catheterized in the jugular vein 1 day before initiation of experiments. Blood samples were taken at 10-min intervals for 6 h, starting 2 h before treatments were applied. Relative concentrations of LH and FSH were calculated by obtaining the ratio of the average concentration of each hormone 2 h after infusion divided by the average concentration during the 2 h before infusion. Relative concentrations of FSH after IGnRH-III infusion did not influence mean concentration of FSH at any of the doses; yet 10.0 μg/kg body weight had a significant effect on LH secretion (p < 0.01). Relative concentrations of LH averaged 1.2, 1.0 and 3.0 ng/ml (for doses of 0.1, 1.0 and 10.0 μg/kg body weight of IGnRH-III respectively). Only a dose of 10 μg/kg body weight elicited a significant LH increase that was associated with exogenous IGnRH-III infusion. We conclude that IGnRH-III is a weak GnRH agonist and at high doses, IGnRH-III has the ability to release LH but not FSH in barrows.