The short-term effects on rates and durations of self-injurious behavior and self-directed stereotypies associated with various doses of fluoxetine (FLX) and venlafaxine (VEN) were examined in rhesus macaques. Adult male macaques (Macaca mulatta; n = 17; age, 7 to 15 y) with at least 1 episode of severe SIB within the past 5 y were randomized to treatment with FLX (n = 6), VEN (n = 6), or placebo (PLC, n = 5), administered by voluntary consumption of medication provided in fruit-flavored tablets. After 4-wk baseline and 4-wk placebo lead-in phases, doses were increased monthly for 4 mo (FLX: 0.5, 2.0, 4.0, and 8.0 mg/kg; VEN: 2.0, 4.0, 8.0, and 16.0 mg/kg). Animals in the PLC condition received similar nonmedicated fruit-flavored tablets. Focal behavioral observations, plasma drug levels, and neurochemical data were obtained. Results indicated that rates and percentage time spent self-biting declined at all doses of FLX, with the greatest effect seen at 2.0 mg/kg. For VEN, percentage time spent self-biting was significantly lower only at the 4.0 mg/kg dose. Treatment-induced reductions in platelet serotonin and cerebrospinal fluid 5-hydroxyindoleacetic acid (CSF 5HIAA) concentrations were substantially greater in the FLX-treated condition than in the VEN-treated condition. Plasma FLX and norfluoxetine levels increased with FLX dose; plasma levels of VEN were low and not dose-related. Fluoxetine at a dose of 2.0 mg/kg daily was most efficacious in reducing SIB, and the observed reductions in platelet serotonin and CSF 5HIAA levels indicated substantial bioeffect at this dose. Treatment with VEN was marked by noncompliance, low bioeffect, and low efficacy.