The purpose of this study is to histologically and immunohistochemically determine the changes created by grayanotoxin-III (GTX-III), which is a sodium channel neurotoxin, on heart tissues in different dosages. Rats were randomly divided into 10 groups to determine the acute and chronic effects of GTX-III. While the rats in groups 1 and 6 were control rats, the rats in groups 2-5 (1, 2, 4, and 8 mu g/kg bw GTX-III) received a single dose of intraperitoneal GTX-III, and the rats in groups 7-10 received GTX-III every day for 3 weeks. As a result of the trial, in the heart tissues, histopathological changes were determined by hematoxylin-eosin staining, interleukin-1 (IL-1 beta), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), and brain natriuretic peptide (BNP) were determined by the avidin-biotin peroxidase method, and apoptosis was examined by immunohistochemistry (IHC) analysis and the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) staining method. In the immunohistochemistry sense, while the BNP level in the AGTX-III groups did not vary significantly, an increase in dosage significantly increased the IL-6, IL-1 beta, and TNF-alpha levels in comparison to the control groups. In their comparison to the control groups, the BNP levels increase and the IL-6 and IL-1 beta levels decreased in the CGTX-III groups. TUNEL analysis revealed that apoptosis increased in both the acute and chronic groups.