1. 1. After intracardial injection, 14C-cycloleucine or 14C-α-aminoisobutyric acid (AIB) rapidly attains a steady-state distribution between host tissues and Hymenolepis diminuta. AIB is accumulated by rat tissues, while cycloleucine is not. 2. 2. Efflux of cycloleucine from Hymenolepis in amino acid mistures resembles that from labeled worms implanted in rats. 3. 3. Feeding of proline to the rat-tapeworm system results in a decrease of worm cycloleucine, while feeding of lysine produces an increase in worm cycloleucine. This is interpreted in terms of demonstrated differences in the cycloleucine absorption systems of Hymenolepis and the rat. © 1969.