The effects of serotonergic and dopaminergic drugs on free swimming behavior in adult sea lampreys (Petromyzon marinus) were investigated using video image analysis. Injections of the serotonin precursor 5-hydroxy-L-tryptophan along with the serotonin reuptake blocker clomipramine into the visceral cavity of lampreys resulted in significant increases in the cycle period of swimming, but had no significant effects on the propagation time of the swim waves down the body (normalized to cycle period), or on the degree of body curvature. Injections of the dopamine agonist apomorphine resulted in significant decreases of cycle period and body curvature with no significant effects on the normalized wave propagation time. The effects on cycle period are consistent with previous findings using serotonin and apomorphine on swimming activity in the isolated spinal cord.