The objective of this study was to investigate the in vitro effects of the organochalcogen 3-methyl-1-phenyl-2-(phenylseleno)oct-2-en-1-one on some parameters of oxidative stress in liver, kidney, and heart of 10-day-old rats. The homogenates of liver, kidney, and heart were incubated for 1 h in the absence (control) or in the presence of 1, 10, or 30 μM of the organoselenium and thiobarbituric acid reactive substances, carbonyl, and the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) were measured. First, we tested the influence of the compound on 1,1-diphenyl-2-picrylhydrazyl (DPPH(•)) radical scavenging and verified that the organochalcogen did not have any antioxidant properties. We observed an increase of lipid peroxidation in all concentrations tested in heart and kidney, while in liver only in the concentrations of 10 and 30 μM. Moreover, we also verified an enhance of protein oxidation in the concentrations of 10 and 30 μM in kidney. On the other hand, the compound caused a reduction on the activity of CAT in heart (10 and 30 μM), liver (30 μM), and kidney (30 μM). The activity of SOD was increased in heart (10 and 30 μM), while in liver (30 μM) and in kidney (10 and 30 μM) the activity was reduced. Our findings indicate that this organoselenium compound induces oxidative stress in liver, heart, and kidney of immature rats, collaborating to the fact that these tissues are potential targets for the organochalcogen action.