Summary Experiments were carried out on female albino (Wistar) rats to establish ricin's liver damaging effect. In accordance with the data in the literature it seems that: 1. 2 μg/kg i.p. ricin (investigated 24 h later of its administration) has a detectable hepatotoxic effect; i.e. electron-density changes of cells and swelling of mitochondria. These findings correspond to the common and first ultra-structural signs of liver cell damage. This result was further strengthened by the fact that serum ALT and AST values were significantly elevated compared to the control value. 2. The next steps of ricin's damaging effect have been detected at 10 μg/kg i.p. dose, — namely: Effect on smooth endoplasmic reticulum: in its place there is a loose, foam-structured unidentified material, — while in the granulated endoplasmic reticulum the number of ribosomes decreased, similarly to the glycogen granules. 3. 200 μg/kg i.p. ricin caused a severe liver-cell damage. The mitochondria showed early degenerative signs, — and both endoplasmic reticulums were further damaged. The most significant feature is the complete lack of ribosomes in the tubular structure of the granulated endoplasmic reticulum. This latter finding enlights the known inhibitory effect of ricin on protein synthesis. The serum enzyme-levels remained in the pathological range. No early sign of enzyme (Cytochrome P 450) induction could be observed.