Spices which show hypoglycaemic, hypolipidaemic and antioxidant activities may have a role in the treatment of diabetes and its complications. The present study aimed to compare the modulatory effects of garlic, ginger, turmeric and their mixture on the metabolic syndrome and oxidative stress in streptozotocin (STZ)–nicotinamide diabetic rats. Diabetes was induced in overnight fasted rats by a single intraperitoneal injection of STZ (65 mg/kg body weight) and nicotinamide (110 mg/kg body weight, 15 min before STZ injection). Diabetic rats orally received either distilled water (as vehicle) or 200 mg/kg body weight of garlic bulb, ginger rhizome or turmeric rhizome powder suspension separately or mixed together (GGT mixture) for twenty-eight consecutive days. The results showed that these spices and their mixture significantly alleviated (80–97 %, P < 0·05–0·001) signs of the metabolic syndrome (hyperglycaemia and dyslipidaemia), the elevation in atherogenic indices and cellular toxicity in STZ–nicotinamide diabetic rats by increasing the production of insulin (26–37 %), enhancing the antioxidant defence system (31–52 %, especially GSH) and decreasing lipid peroxidation (60–97 %). The greatest modulation was seen in diabetic rats that received garlic and the GGT mixture (10–23 % more than that in the ginger and turmeric groups). In conclusion, garlic or the mix including garlic appears to have an impact on each of the measures more effectively than ginger and turmeric and may have a role in alleviating the risks of the metabolic syndrome and cardiovascular complications.