The effects of metoclopramide (MCP) and bromocriptine (BC) on the growth of neuroblastoma (NB) cells and their influence on the plasma membrane binding of several neurotransmitters were studied. In the first part of this study, in vitro experiments were done with three human and two murine NB cell lines. Dibutyryl cyclic 3',5'-adenosine monophosphate is known to differentiate NB cells in vitro and served as a reference substance during the experiments. MCP significantly reduced the replication rate in NB cells and increased cellular differentiation by morphological as well as by functional criteria. BC, in contrast, stimulated cell replication. Similar to dibutyryl cyclic 3',5'-adenosine monophosphate, MCP increased the binding capacity of the plasma membrane for the beta-adrenergic hormones dopamine and noradrenaline. In the second part, the effects of BC and MCP upon NB tumor growth were investigated in vivo in the mouse. Significant changes in tumor growth were induced; BC promoted and MCP inhibited the NB tumor growth in a dose-dependent relationship. The findings are discussed, along with the observed accompanying changes in serum copper and in the peripheral blood count.