The innervation of the trabecular meshwork of the anterior chamber angle in monkey eyes was studied by transmission and scanning electron microscopy. According to the transmission electron microscopic examination, the nerve terminals in the trabecular meshwork mostly contained dense core vesicles after treatment with 5-hydroxydopamine and thus were identified as the adrenergic nerve terminals. Regarding the mode of ending of the nerve terminals in the trabecular meshwork, it was divided into three types from the positional relationship of the nerve terminals with the endothelial cells in the trabecular meshwork. Type I nerve terminals are naked and they form synaptic contact with endothelial cells of the trabecular sheets of the uveal and corneoscleral meshwork. Type II nerve terminals are located in the trabecular sheets of the uveal and corneoscleral meshwork and in the endothelial meshwork, and partly surrounded by the Schwann cell processes. This type of nerve terminals end as free terminals without a constant relation to endothelial cells of the trabecular meshwork. Type III nerve terminals usually not surrounded by the Schwann cell processes are observed on the free surface of the endothelium of the trabecular sheets facing the aqueous humor. The incidence of these three types of nerve terminals was the highest in type II followed by type III. Type I nerve terminals were very rare. When the morphological characteristics of the trabecular meshwork are considered, the transmitter released from type II and type III nerve terminals enter the aqueous humor and perhaps affects the aqueous outflow area to control the intraocular pressure, as judged from the flow of aqueous humor.(ABSTRACT TRUNCATED AT 250 WORDS)