The main interest of the present study was to determine possible alterations in β-endorphin serum levels in healthy volunteers and in patients with depression, as well as changes in β-endorphin serum levels caused by fluvoxamine treatment. Fluvoxamine maleate (Fevarin®) was administered orally at a dose of 200 mg/day for 4 weeks. The serum levels of β-endorphin were lower in patients with ‘nonendogenous’ depression (104.68±5.29 pg/ml) and those with ‘endogenous’ depression (36.34±2.23 pg/ml) than in healthy volunteers (125.19±1.64 pg/ml). The endogenously depressed patients had significantly lower β-endorphin levels than the nonendogenous patients. A 4-week treatment of fluvoxamine (200 mg/day) caused a statistically significant increase in β-endorphin serum levels in all patients (nonendogenous depression 132.10±2.38 pg/ml and endogenous depression 50.09±2.45 pg/ml) in comparison to values found before the onset of the therapy. The efficacy of fluvoxamine was 11.0 (±9.0) evaluated by the Hamilton Rating Scale for Depression (HAMD) in the patients with a diagnosis of depression. These results indicate that determination of β-endorphin serum levels could be a valuable laboratory test in the diagnosis of depression.