The motor responses (such as stereotypic behavior or convulsions induced in rats by N-methyl-D-aspartate (NMDA) administered systemically were followed by a rapid, moderate increase in the putrescine concentration in plasma which preceded an increase in this amine in the brain. This effect was not observed following the convulsions evoked by pentylentetrazol, picrotoxinine, lindane or 4-aminopyridine. However, all the convulsants assayed induced a mild increase in the concentration of putrescine in the frontal cortex and hippocampus. A differential activation of the ornithine decarboxylase (ODC)/polyamine system in both cerebral and peripheral tissues could account for these results.