The aim of this study was to examine the effects of the potent schistosomicide oxamniquine on the testicular morphology of adult guinea pigs following incorporation of the drug into synthetic phospholipid–DMPC vesicles. The guinea pigs were allocated to one of four groups: a) controls (no vesicles or oxamniquine), b) vesicletreated controls which received DMPC (80 mM), c) treatment with oxamniquine (15 mg/kg) incorporated into DMPC, and d) treatment with oxamniquine diluted in mineral oil. Morphological evaluation showed that the testicular structure in the groups treated with DMPC vesicles and with oxamniquine incorporated into DMPC was similar to the control group. However, guinea pigs that received only oxamniquine had testes with a variety of morphological alterations. These findings indicate that when oxamniquine was administered alone it caused testicular damage, but when incorporated into synthetic phospholipidic vesicles, the side effects were suppressed.