The action of phenylalanine derivatives against a cyst forming strain of Toxoplasma gondii was tested in vitro and in vivo in mice. These compounds were Phe-Phe-OMe (dipeptide methyl ester) 1 and its cyclized product, 3,6-dibenzyl-2,5-dioxopiperazine 2, Boc-L-Phe 3, L-Phe-OMe 4, Boc-L-Phe-L-Phe-OMe 5. After a 48 hr incubation in vitro, the compounds 3 and 5 induced a higher inhibition than the control molecule, pyrimethamine. In the in vivo studies, the compound 3 induced a 77% decrease in the number of cerebral cysts, comparable to pyrimethamine. Compounds 1, 5 and 4 induced a decrease of about 63% in the cyst number. A size reduction and an alteration of the wall of treated cysts were often noted. In a histological study, a reduction in cyst size without either inflammation or intervention of the nevroglial cells was observed. The present study provides evidence on the efficacy of phenylalanine derivatives and especially Boc-Phe 3, against T.gondii brain cysts in mice.