Nicotine induces profound behavioral responses in the model organism Caenorhabditis elegans. We tested the effect of a broad range of concentrations of nicotine (from 0.001mM to 30mM in nematode growth medium) on C. elegans locomotor behavior. We also followed the time-course influence on the sensitivity of C. elegans to nicotine (from 0min to 300min). A low concentration (0.001mM) of this alkaloid causes a reduction of the speed of movement. By contrast, moderate concentrations (0.01 and 0.1mM) induced acceleration of the mean speed of locomotion of C. elegans. High doses of nicotine (above 1mM) induced slowing down of the movements and, finally, paralysis. Time-dependent analysis revealed that the stimulating effect of nicotine abolished the slowing down of C. elegans in control experiments after 30min in the presence of 0.001, 0.1 and 10mM nicotine. In the presence of 0.1mM nicotine, the stimulation phase lasted up to 70min. The evidence indicates that nicotine can have dual effects on the speed of locomotion, which is dependent on differences in its dosage and treatment time.