Abstract: Background and Objective: Green synthesis of nanoparticles is gaining wide acceptability because of their acclaimed lesser toxicity. This study was aimed at synthesizing manganese oxide nanoparticle from Cassia tora aqueous leave extract in order to investigate the toxicological effect of the biologically synthesized manganese oxide nanoparticles (MnO2 NPs). Materials and Methods: The biosynthesized MnO2 NPs was characterized using UV-visible spectrometry, Fourier Transform Infrared (FTIR) spectroscopy and scanning Electron Microscope (SEM). Twenty adult albino rats were randomly divided into four groups, group 1 served as the control while groups 2, 3 and 4 received orally 100, 200 and 400 mg kg–1 b.wt., Cassia tora leaves biosynthesized MnO2 NPs, respectively for 21 days. Haematological, liver function and kidney function parameters were carried out. Results: X-ray diffraction revealed that the biosynthesized MnO2 NPs was spherically agglomerated in nature while the particle size was 75 nm. There was no significant difference in Red Blood Cell (RBC), Hemoglobin (Hb), Packed Cell Volume (PCV), White Blood Cell (WBC), Alanine Aminotransferase (ALT), Aspartate Aminotransferase (AST), Alkaline Phosphatase (ALP), bilirubin, urea, creatinine, Na+ and HCO3– values in rats orally administered 100, 200 and 400 mg kg–1 b.wt., biosynthesized MnO2 NPs when compared with the control group. However, total protein significantly decreased while K+ concentration significantly increase. Conclusion: This indicates that biosynthesized MnO2 NPs from Cassia tora leaves is not completely safe at these doses tested.