Cilia in the pigmented and nonpigmented ciliary epithelial cells were studied by electron microscopy in the normal Rhesus monkey eye. In addition cilia formation was studied following paracentesis as well as injection of hemoglobin into the vitreous cavity. In all eyes studied, cilia had a 9 + 0 microtubule pattern; however, some cilia had aberrant arrangement of microtubules. Cilia increased in number and showed elongation in the treated eyes, these newly-formed cilia occasionally extending towards the cell interior opposite to the normal orientation. The results suggested that the formation of ciia can be provoked by a variety of experimental stimuli either with or without drugs.