BACKGROUND: The covalent modification of interferon (IFN) α2b with monomethyoxy polyethylene glycol (PEG) reduces its clearance rate and increases its half-life. High doses of interferon (IFN) α2b have previously been shown to affect maintenance of pregnancy in rhesus monkeys. Given the role of ovarian hormones in reproductive function and pregnancy, this study was conducted to assess the effects of PEG-IFNα2b or IFNα2b (comparative control) on ovarian hormones and menstrual cyclicity in cynomolgus monkeys. In addition, the potential for reversibility of PEG-IFNα2b or IFNα2b-related observations was assessed. METHODS: Monkeys were administered 3,105 µg/m2 human recombinant (hr) IFNα2b or 52, 262, or 4,239 µg/m2 PEG-hr-IFNα2b every other day for one menstrual cycle, followed by a post-dose period of up to two menstrual cycles. RESULTS: Monkeys administered 3,105 µg/m2 hr-IFNα2b or 52, 262, or 4,239 µg/m2 PEG-hr-IFNα2b exhibited transient decreases in food consumption, leukocyte and erythrocyte parameters. Monkeys administered 3,105 µg/m2 hr-IFNα2b exhibited lengthened menstrual cycles that were associated with a delay in reaching peak ovarian hormone levels and lower respective peak concentrations. Similarly, monkeys administered 4,239 µg/m2 PEG-hr-IFNα2b exhibited lengthened menstrual cycles and a delay in reaching peak ovarian hormone levels and slightly lower respective peak concentrations. Post-dosing menstrual cycle length, estradiol and progesterone profiles exhibited evidence of recovery in both the hr-IFNα2b and the high-dose PEG-hr-IFNα2b groups. CONCLUSIONS: Administration of hr-IFNα2b or PEG-hr-IFNα2b at high doses to cynomolgus monkeys resulted in similar effects on menstrual cycles, estradiol and progesterone profiles, and exhibited evidence of reversibility upon cessation of dosing. These results suggest that the previously observed high-dose IFNα-related effects on the maintenance of pregnancy in monkeys are likely the result of altered ovarian function. Birth Defects Res (Part B) 86:29-39, 2009. © 2009 Wiley-Liss, Inc.