The pathophysiology and treatment of depression involves monoamine neurotransmitters and the magnesium (Mg)-modulated monoaminergic pathway. Serum and platelet Mg concentrations and platelet serotonin concentrations were measured in 79 depressed patients who had attempted suicide, and 101 patients without suicidal behaviour, according to the ICD-10 diagnoses F 33.2 and F32.2, with or without intentional self-harm (X60-X84). The control group consisted of 77 voluntary blood donors. The platelet serotonin concentration was determined using the competitive enzyme immunoassay test: Mg concentrations in platelets and serum were determined by atomic absorption spectrophotometry. The ANOVA test showed significantly lower serum Mg in the group of depressive patients who had attempted suicide (N = 257, F = 8.32, p<0.001), compared to depressive patients who had not, and the control group. Serum albumin was lower in the group of depressive patients who had attempted suicide and showed a significant, positive correlation with serum Mg concentrations. Platelet Mg concentrations were found to be higher in depressive patients who had not attempted suicide (N = 257, F = 3.90, p = 0.012) compared to the control group, with no difference compared to depressive patients who had attempted suicide. The Kruskal Wallis test (N = 257, H = 48.54, p<0.0001) showed the lowest concentration of platelet serotonin in the groups of depressed patients with and without suicidal behaviour, compared to the healthy control group. A positive correlation was found between platelet Mg and serotonin concentrations only in the healthy control group. In conclusion, differences were observed in serum and platelet Mg concentrations, which represent progress in the study of Mg status and its relation to serotonin.