The effects of i.v. 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on renal and cardiac sympathetic nerve activity, phrenic nerve activity, blood pressure and heart rate were investigated in urethane and pentobarbitone anaesthetised rabbits. Cumulative doses of 8-OH-DPAT (0.5-512 micrograms kg-1) in urethane anaesthetised rabbits, in which simultaneous recordings of renal and phrenic nerve activity were made, caused dose related decreases in mean blood pressure which were maximal after 32 micrograms kg-1 and dose related decreases in heart rate. Effects on renal and phrenic nerve activity were variable. A single dose of 8-OH-DPAT (32 micrograms kg-1) in either urethane or pentobarbitone anaesthetised rabbits, in which simultaneous recordings of cardiac as well as renal and phrenic nerve activity were made, caused significant falls in blood pressure, heart rate and cardiac nerve activity. Renal nerve activity showed no significant change whilst phrenic nerve activity tended to increase but this was only significant in the pentobarbitone anaesthetised rabbits. In all experiments the bradycardia produced by 8-OH-DPAT was unaltered by atropine. These results show that in the anaesthetised rabbit the hypotension and bradycardia caused by 8-OH-DPAT were associated with a decrease in cardiac nerve activity, increased central respiratory drive, and no change in renal nerve activity or vagal tone.