The in vitro uptake of nicotine into the crude synaptosomal fraction of rat brain and spinal cord was studied. The tissue/midium ratio was low and the changing of incubation time or [14C]nicotine concentration did not affect the ratio, nor did a metabolic inhibitor, sodium fluoride. A lowered ratio was obtained at 0degrees C, but this decrease may be attributable to an altered pKa of the drug at low temperature. Nicotine antagonists, mecamylamine and hexamethonium, did not affect the ratio when incubating the crude synaptosomal fraction of either adult or infant rat brain. These results suggest that the uptake of nicotine into the synaptomal fraction is not an active process. When mecamylamine and nicotine were injected in vivo, the mecamylamine antagonism was also demonstrated as lowered nicotine concentrations in infant ray synaptosomes. Since the newborn rat cortex lacks glial tissue, the nicotine concentrations in the crude synaptosomal fraction of infant rats may reflect the receptor level effects better than in adult brains. The pretreatment of infant rats with mecamylamine also lowered blood nicotine levels, suggesting that mecamylamine affected nicotine brain levels also in an unspecific way.