Polycystic ovary syndrome (PCOS) is one of the most common ovarian diseases among women of reproductive age. The reproductive and metabolic traits of PCOS are underpinned by adipocyte dysfunction, especially diminished adiponectin secretion. Based on evidence that niacin stimulates adiponectin secretion, this study evaluated the effects of niacin on adiponectin concentrations and reproductive traits in a rat model of PCOS. PCOS was induced by single injection of 4mg kg-1 oestradiol valerate (i.m.), and PCOS groups were administered orally with saline or niacin (10 or 25mg kg-1) daily for 30 days after PCOS induction. The control group received 0.2mL sesame oil (i.m.) only. At the end of the experimental period, serum samples and ovaries were collected for adiponectin, histological and molecular analyses. Niacin reduced the bodyweight gain and increased ovary weights in PCOS rats. Niacin also increased the number of normal antral follicles and corpora lutea while reducing the number of cystic follicles and the thickness of theca interna. Moreover, niacin significantly increased serum adiponectin concentration and the gene expression of adiponectin and its type 1 receptor. In conclusion, this study indicates that niacin reduces cystic follicles and improves ovulation in PCOS rats. Adiponectin signalling may have contributed, in part, to the beneficial effects.