The authors studied the effects of clonidine on a subgroup of women who had symptoms associated with premenstrual syndrome; the subgroup comprised 24 women aged 19 to 41 years who had "moderate" to "severe" cyclic decreases in beta-endorphin levels. All of the women received clonidine and placebo in a double-blind cross-over design that spanned four menstrual cycles. Clonidine was significantly (p less than .05) more effective than placebo in reducing symptoms.