Helichrysum species (Asteraceae) are widely found in Anatolia. Decoction prepared from the capitulums of Helichrysum plicatum ssp. plicatum is used to alleviate the symptoms of diabetes mellitus in folk medicine. In the present study, the hypoglycaemic and antioxidant potential of Helichrysum plicatum ssp. plicatum was evaluated by using in vivo methods in normal and streptozotocin-induced-diabetic rats. After the oral administration of water and ethanolic extracts at doses of 500 mg/kg body weight prepared from the capitulums of plant, blood glucose levels were monitored at specific intervals. Tolbutamide was used as a reference drug at a dose of 100 mg/kg. The experimental data indicated that water and ethanol extracts of capitulums demonstrate significant antihyperglycaemic and antioxidant activity in streptozotocin-induced rats which confirmed the folkloric utilization. In order to assess the role of polyphenolic components in the relevant activity, phenolic and flavonoid contents of each extract were also determined in terms of total phenols: 113.5 +/- 8.6 mg (gallic acid equivalent/l g extract) and total flavanoids 50.5 +/- 1.9 mg (quercetin equivalent/l g extract) for ethanol extract, total phenols: 75.9 +/- 3.7, flavonoids: 31.5 +/- 2.3 for water extract using Folin-Ciocalteu reagent. (c) 2006 Elsevier Ireland Ltd. All rights reserved.