K-II is an analogue of U-50488, a selective kappa-opioid agonist. Comparison of pharmacological profile of K-II and U-50488 was studied using in vitro and in vivo methods. The results showed that the IC50 values of K-II and U-50488 on electrically induced contraction of the rabbit vas deferens were 0.42 nmol/L and 26.5 nmol/L, respectively. ED50 values of K-II and U-50488 in impairing motor function of mouse in the horizontal screen test were 1.7 and 15.3 mg/kg, respectively. The effect of K-II in reducing spontaneous activity of mouse was 6 times as potent as U-50488. These results suggest that K-II is a more potent kappa agonist in pharmacological effects than U-50488.