The aim of this study was to assess the effects of diarrhea on hematocrit and various serum biochemical parameters (serum electrolytes, trace and ultra-trace elements) in foals. A total of 105 diarrheic foals (n = 35 horse foals, n = 35 donkey foals, and n = 35 mule foals) were selected regardless of their etiologies. Additionally, 12 healthy foals (n = 4 horse foals, n = 4 donkey foals, and n = 4 mule foals) as control animals were selected for this study. Packed cell volume (PCV) and serum electrolytes, trace and ultra-trace elements were measured in both groups (diarrheic and healthy). The results showed significant (p< 0.05) increase of PCV% in diarrheic foals of all species. The sodium, calcium, copper, iron and lithium concentrations significantly (p < 0.05) decreased while the potassium concentration significantly (p < 0.05) increased in diarrheic foals. Hence, this can be concluded that diarrhea in foals have significant correlation with PCV, serum electrolytes, and trace and ultra-trace elements.