Planarians can be used as invertebrate bioassays to evaluate the role of neurotransmitters on regenerating cells. The influence of the nervous system is crucial to regenerate a normal complete animal. The neurotrophic action of the nervous system has been attributed to the major neurohormones present throughout the animal kingdom. The same type of transmitters found in mammals have been extensively found in many invertebrates, including planarians, but their role in regeneration is unclear. Neurotransmitters and drugs which act on neurohumoral transmission have been used to determine the role of each neurohormone on regenerating planarians. Biochemical and pharmacological mechanisms of neurohormones on regenerative planarians are reviewed, as is their putative role on regeneration. Correlations with the roles of neurotransmitters in the central nervous system of higher organisms are also addressed.