Nicotinamide is an important cofactor in many metabolic pathways and a known neuroprotective substance, while its methylated product, 1-methylnicotinamide, is a suspected neurotoxin. Homocysteine is a risk factor in Alzheimer's disease and neurodegeneration, causing inhibition of methylation processes and inducing excitotoxicity. In this study, using primary cultures of rat cerebellar granule cells and propidium iodide staining, we investigated the neurotoxicity of nicotinamide and 1-methylnicotinamide, and their neuroprotective potential in acute and sub-acute homocysteine neurotoxicity. Our results demonstrated that nicotinamide and 1-methylnicotinamide applied for 24 h to cultures at concentrations of up to 25 mM had no effect on neuronal viability. Moreover, nicotinamide at concentrations of 5?20 mM and 1-methylnicotinamide at 1?10 mM applied to cells 24 h before, and for 24 h after an acute 30 min application of 25 mM D,L homocysteine, reduced neuronal damage. 1-Methylnicotinamide at concentrations of 250 and 500 ?M showed neuroprotective activity during a sub-acute 24-h exposure to 2.5 mM D,L-homocysteine, while 5 and 25 mM nicotinamide also evoked neuroprotection. These findings do not support suggestions that 1-methylnicotinamide may act as an endogenous neurotoxic agent; rather, they indicate the neuroprotective ability of nicotinamide and 1-methylnicotinamide in homocysteine neurotoxicity. The exact mechanisms of this neuroprotection are unclear and require further investigation.