In this study we examined the effects of 3-24 h of incubation of chemically skinned rat fast-twitch muscle with the glycolytic metabolite glucose 6-phosphate (G6-P) on the contractile properties and myosin ATPase activity in single muscle fibres, and on the carbohydrate content of myosin heavy chains (MHCs). Exposure of the permeabilised muscle to 10 mM G6-P for 24 h at 22+/-1 degrees C in a rigor solution containing protease inhibitors and a reducing agent (dithiothreitol, DTT) significantly decreased maximum Ca(2+)-activated force output by 31%, lowered the Ca2+ threshold for contraction by 0.1 pCa units and produced shallower force-pCa curves compared with controls. Furthermore, under these conditions, G6-P-treated muscle displayed lower myofibrillar MgATPase activity and a markedly higher carbohydrate content of MHCs, as identified with an immunoblot protocol for glycoprotein detection. Shorter incubations under the same conditions or 24-h incubations with 5 mM G6-P generally resulted in smaller changes in the contractile activation parameters. These findings suggest that reducing sugars acting as metabolic intermediates in the glycolytic pathway can have important non-energy-related effects on the contractile activation characteristics of mammalian skeletal muscle. These effects are consistent with the glycation of muscle proteins, in particular that of the MHC.