The aim of this study was to assess the effects of DL-homocysteine (DL-Hcy) and DL-homocysteine thiolactone (DL-Hcy TLHC) on selected serum biochemical parameters, markers of oxidative stress and the activities of antioxidant enzymes (catalase (CAT), glutathione peroxidase (GPx), superoxide dismutase (SOD)) in the plasma, as well as on acetylcholinesterase (AChE) activity in the cardiac tissue homogenate in the rat. Male Wistar rats were divided into three groups as follows: control group (1 mL 0.9% NaCl, intraperitoneal (i.p.) injection), DL-Hcy group (8 mmol/kg body mass (b.m.), i.p.) or DL-Hcy TLHC group (8 mmol/kg b.m., i.p.). One hour after administration, the rats were euthanized, whole blood was collected for biochemical analysis, and the heart was excised. Following the i.p. administration of DL-Hcy and DL-Hcy TLHC, the activities of antioxidant enzymes were mostly significantly increased, while plasma malondialdehyde (MDA) was decreased. Administration of DL-Hcy and DL-Hcy TLHC significantly inhibited AChE activity in rat cardiac tissue. Our findings suggest that DL-Hcy and DL-Hcy TLHC exerted prooxidant effects; however, the decrease in MDA points to an inverse response to the increase in antioxidant enzyme activities. While both substances inhibited AChE activity in rat cardiac tissue, DL-Hcy TLHC induced stronger effects than DL-Hcy.