The ultrastructure of 145 pituitary gonadotroph adenomas was investigated with special regard to the fine structural appearance of the Golgi apparatus. Eight-five of the tumors were removed from men, 60 from women. Except for a few males, gonadotropin oversecretion was not established clinically and the diagnosis had been made solely on morphologic grounds. Owing to sex-related dimorphism, the diagnostic criteria were different for tumors of men and women. In men, approximately 50% of adenomas were structurally well-differentiated, whereas the rest of the tumors consisted of areas showing varying degrees of differentiation. The Golgi complex exhibited no signs of vacuolar transformation in any of the tumors in men. In women, more than 70% of tumors were uniformly well-differentiated, displaying vesicular transformation of Golgi complex (honeycomb Golgi complex), a characteristic diagnostic marker. In two adenomas, instead of the honeycomb Golgi complex, another unique alteration of the Golgi apparatus was encountered, comprising transformation of Golgi complex into a tripartite cloverleaflike structure with a pair of centrioles at its base. As suggested by formation of vacuoles in the center of the loops, this unusual change may be the fore-runner of honeycomb Golgi complex. Differences of the Golgi complex are viewed as morphologic expression of the fact that gonadotroph regulation and function in the two sexes are not the same.