Spirulina platensis (Spi) is a microalga presenting high contents of proteins, γ-linolenic acid, vitamins and minerals, and showing many biological activities. It is a promising drug for the treatment of diseases including diabetes. The objectives of this work were to study Spi effects on alloxan-induced diabetic rats, and associate this to its anti-inflammatory activity. The treatment with Spi (25, 50 or 100 mg/kg, p.o.) started 48 h after the alloxan injection, continuing for 5 or 10 days. Biochemical parameters were measured in sera of treated and untreated animals. The anti-inflammatory activity of Spi was assessed by the formalin test and carrageenan-induced paw edema in mice. Immunostainings for TNF-alpha were carried out in the carrageenan-induced paw edema in rats, before and after the Spi treatment, and its effect on the release of myeloperoxidase from human neutrophils was also determined. Spi decreased glycemia as well as triglyceride and total cholesterol levels of diabetic rats. Levels of urea and creatinine were also reduced, while liver transaminases were unaltered. Spi also decreased dose-dependently the 1st (neurogenic) and mainly the 2nd phase (inflammatory) of the formalin test, as well as the carrageenan-induced paw edema in mice. The anti-inflammatory effect of Spi was further confirmed by decreases in TNF-alpha immunostaining in the inflamed paw and in the myeloperoxidase release from human neutrophils. The results showed that the anti-diabetic effect of S. platensis is already manifested after a 5-day treatment. Additionally, considering the relationship between diabetes and inflammation, the microalga anti-inflammatory action may also be involved.