Two 2-(di-n-propylamino)tetralin (DPAT) compounds, 8-OH-DPAT and 5-OH-DPAT, with reported effects on central 5-HT and DA receptors respectively, were tested for their effects on the acoustic startle response in rats. 8-OH-DPAT was given in doses of 0.25-2.0 mg/kg IP and 5-OH-DPAT in doses of 1.0-8.0 mg/kg IP. Both compounds increased the startle response significantly in a dose-dependent manner, but 8-OH-DPAT appeared to be about 30 times as potent and to have a higher efficacy than 5-OH-DPAT. In addition, the effects on the startle response of L-5-HTP, 25-100 mg/kg IP, and L-dopa, 25-100 mg/kg IP, administration to animals pretreated with the inhibitor of aromatic L-amino acid decarboxylase, benserazide (25 mg/kg IP) were included for comparison. A small, but significant increase in the startle amplitude was found after the highest dose of L-5-HTP, whereas no effects were observed after L-dopa administration.