Effect of ethanolic extract of Buplerum marginatum has been studied on the periodicity of oestrous cycle in adult rats. It is found that the extract at a dose of 50 mg/kg/day when given for 10 days caused significant increase in the duration of metaestrus phase of the oestrous cycle. Other longer durations of treatment also caused similar changes. Percent increase in the duration of metastrus phase at 10,20 and 30 days has found to be identical. No significant effect on body weight was recorded during and after the treatment with extract. Irregularlity in the oestrous cycle of treatment with extract. Irregularity in the oestrous cycle of treated rats resumed to normal after discontinuation of treatment, however, the period of recovery depended on the dose administrated.