Abstract: According to the quantitative structure-activity relationship studies of 3-pyrazolidi-nones with different substituent on positions 1 and 5 reported previously, the anticonvulsant activity isparabolically related with the total fragment constent ( Fr hydrophobic parameter) of the 1 and 5 sub-stituents of 3-pyrazolidion. The optimum Fr was about 5. 6. Therefore, eleven new 5-substituted-3-pyrazolidinones have been synthesized. Pharmacological test showed that they are all potent anticonvul-sant agents. Among them 1-n-butyl-5-(p-chlorophenyl)-3-pyrazolidinone was shown to be the mostpotent so far synthesized.