Spermiating striped bass, a perciform fish, were treated with two controlled-release gonadotropin-releasing hormone agonist (GnRHa)-delivery systems, and the resulting changes in plasma gonadotropin II (GTH II), testosterone (T), 11-ketotestosterone (11-KT), 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P), and 17,20beta,21-trihydroxy-4-pregnen-3-one (17,20beta,21-P) were correlated with changes in milt production and sperm density. GnRHa-delivery systems induced a sustained elevation of plasma GnRHa and GTH II for 14 days. Plasma T levels were unchanged after GnRHa treatment, while 11-KT levels increased significantly. Plasma 17,20beta-P also increased after GnRHa treatment and remained elevated compared to levels in controls, while plasma 17,20beta,21-P levels were unaffected. Both GnRHa-delivery systems induced many-fold increases in total expressible milt, lasting throughout the 14 days of the study. Sperm density decreased 2 days after GnRHa treatment, with a subsequent increase by Day 7. This study demonstrates that GnRHa-delivery systems induce a sustained elevation of plasma GTH II levels in striped bass, resulting in a long-term enhancement of milt production. The endocrine changes observed suggest that 11-KT and 17,20beta-P, but not 17,20beta,21 -P, are mediating the effects of GTH II on spermiation.