Cyproconazole (CPZ) is a well-established triazole fungicide derivative applied to protect agricultural crops against various types of fungal diseases. The present investigation aims to assess its harmful influences on the adrenal cortex of adult rats and the probable protective impact of the oil extract of fennel (Foeniculum vulgare) against the devastated effects generated by CPZ. Sixty male rats were randomly divided into six experimental groups: the control, fennel oil, CPZ-low dose, CPZ-high dose, fennel oil + CPZ-low dose, and fennel oil + CPZ-high dose treated groups. After 15 days, the adrenal glands from all rats were removed, fixed in the adequate fixative, and processed for histological and ultrastructural examination. The current histopathological results showed that CPZ at the low and high doses significantly induced degenerative changes in the adrenal cortical tissues of the treated rats, as evidenced by reduction in the thickness of the adrenocortical zones; zona glomerulosa, zona fasciculata and zona reticularis, which resulted from the compression of the adrenocortical cells that suffered from cytoplasmic vacuolation and nuclear pyknosis, karyorrhexis and karyolysis, besides, congested and dilated cortical blood vasculatures. The ultrastructural observations showed damaged mitochondria, proliferated smooth endoplasmic reticulum, and increased lysosomes, as well as noticeable aggregation of lipid droplets. While there was an improvement in most of the hazardous impacts induced by CPZ on the histological and fine structural features of the adrenocortical tissues of rats treated with fennel oil extract alongside CPZ. In conclusion, this study evidenced that fennel oil co-treatment has obvious protective role against CPZ-triggered adrenal cortex injury in adult rats owing to its strong antioxidant and anti-inflammatory capacity.