The hypoglycemic activity of Psacalium decompositum (Asteraceae) was investigated in fasting healthy mice and alloxan-diabetic mice. The freeze-dried water decoction significantly reduced the blood glucose in normal mice (from 50.9 +/- 4.7 to 32.5 +/- 3.1 mg/dl) and in mild diabetic mice (from 208.5 +/- 13.0 to 52.3 +/- 7.0 mg/dl), 240 min after intraperitoneal administration (P < 0.005). This preparation also diminished fasting glycemia in severe diabetic mice, but the effects were minor (from 394.4 +/- 9.4 to 289.3 +/- 39.5 mg/dl). The main sesquiterpenoid constituents from P. decompositum roots, cacalol, cacalone and maturin, as well as the transformation product cacalol acetate, did not show a hypoglycemic effect on healthy mice. Nevertheless, two polysaccharide fractions (F1 and F3) obtained from the freeze-dried water extract significantly reduced the fasting glycemia in healthy mice. The best results were obtained with the F1 fraction.