The origin, course and regions supplied by the ethmoidal arteries were studied on 30 injected adult heads. After branching off, the anterior ethmoidal artery normally makes a single smooth loop by first coursing forwards and then, reversing itself towards the anterior ethmoidal foramen, it goes into the canal portion, likewise without bend or angularity. Occasionally, a common ethmoidal artery or a common source for the ethmoidal arteries is present. Very rarely does the artery fail entirely. As a rule, the posterior ethmoidal artery arises from the ophthalmic artery. Occasionally, however, it is missing or can even very rarely arise from the A. meningea media. The artery usually crosses over the superior oblique muscle while the anterior ethmoidal artery usually goes under the same muscle. In the fossa olfactoria, the ethmoidal arteries give off their most important dura and bone branches in the anterior cranial fossa and then continue into the walls of the cavum nasi. The courses and variations along with ipsi- and contralateral anastomoses are likewise demonstrated.