Experiments were conducted to determine the effects of male-male agonistic encounters on changes in monoamine neurotransmitter concentrations in the supraesophageal ganglion (brain) of the tarantula, Aphonopelma hentzi. Serotonin levels were significantly reduced 30 min after fighting in both dominant (66.5 ± 9.1 SE nmol/mg protein) and subordinate (42.8 ± 7.6) animals as compared to isolated controls (89.7 ± 13.2), and these differences persisted for up to 24 h. A similar decrease was found for octopamine concentrations in dominant (43.7 ± 7.7) and subordinate (31.2 ± 4.9) spiders when compared to controls (56.9 ± 5.8). In addition, serotonin and octopamine levels were significantly lower in subordinate vs. dominant spiders. Agonistic interactions had no effect on the concentrations of dopamine, norepinephrine, and epinephrine. In isolated control spiders, serotonin (89.7 ± 13.2 SE nmol/mg protein) was present in highest concentration in the brain, followed by octopamine (56.9 ± 5.8 nmol/mg), dopamine (22.4 ± 3.8 pmol/mg), norepinephrine (15.3 ± 4.7 pmol/mg), and epinephrine (0.57 ± 0.2 pmol/mg). The results indicate that following agonistic encounters, monoamine concentrations in the brain decrease to different levels in winners and losers. This is the first demonstration that the establishment of social status causes changes in brain monoamines in spiders.