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The aim of the present study was to compare the effects of full-length rat kisspeptin (rKp-52) with C-terminal decapeptide (Kp-10) of rat or human kisspeptin on LH release in intact male rats. Plasma LH profiles were determined by frequent blood sampling at 6-min intervals for 3 h after central or peripheral injection of kisspeptins. Intracerebroventricular (icv) injection of rKp-52 (0.1 nmol) induced a gradual increase in the plasma LH level, which remained high for the rest of the sampling period. On the other hand, icv injection of rKp-10 did not increase the plasma LH level at the same dose (0.1 nmol). A 10-times higher dose (1 nmol) of rKp-10 and hKp-10 increased the plasma LH level, but the increase was lower than that of rKp-52 icv injection. Intravenous (iv) injection of kisspeptins also stimulated LH release at 10 or 100 nmol/kg. In rKp-52 (10 nmol/kg)-treated animals, the plasma LH level reached a peak within 30 min and remained high until 60 min postinjection. The rKp-10- and hKp-10-injected animals showed a more rapid decline in plasma LH level after the peak found at around 30 min after the injections at both middle (10 nmol/kg) and high (100 nmol/kg) doses. The present study indicates that full-length kisspeptin is more effective in stimulating LH release compared with Kp-10 in male rats. The difference in LH-releasing activity may be the result of a difference in degradation of the peptides, but it is still worth determining whether an active domain other than the C-terminal decapeptide is present in full-length kisspeptin.