Adults ofPhilophthalmus gralli were subjected to deionized water at room temperature and 3 C. Destruction of the stages of spermatogenesis in the testes was observed after 3 hr of exposure at room temperature. Complete regrowth of normal stages of spermatogenesis was found in worms transplanted back to the host for 10 days. The ability of digenetic trematodes to regenerate complete body parts and organs is almost nonexistent when compared to the turbellarians. Previous studies have indicated that, outside of wound healing, the digenetic trematodes have little regenerative ability (Fried and Penner, 1964; Fried et al., 1971; Austin and Fried, 1972; Bentley, 1982; Popiel et al., 1985). Reproductive organs of digenetic trematodes are the most sensitive tissues to suboptimal conditions and are the first to be reabsorbed or destroyed. Nollen and Alberico (1972) suggested that the poor nutritional conditions of frogs stored in holding facilities before sale by commercial suppliers caused their bladder flukes to lose their testes. The suboptimal nutritional conditions of in vitro culture media for schistosomes caused progressive loss of ovaries, testes, and vitelline glands (Michaels, 1969; Floyd and Nollen, 1977; Kolzow and Nollen, 1978; Irie et al., 1987). Exposure of adults to deionized water will also disrupt the testes as was found in a study with the eyefluke, Philophthalmus gralli (MacNab and Nollen, 1987). Can the reproductive organs initiate production of the stages of spermatogenesis when the flukes are returned to normal nutritional and osmotic conditions, or is the damage permanent? When pairs of Schistosoma mansoni grown in vitro for long periods of time and with impaired reproductive systems were transplanted to mice, they regained the ability to produce normal eggs (Basch and Humbert, 1981). This indicated that the male and female could regenerate reproductive tissues or recover from a damaged condition. The testes of P. gralli are severely disrupted with no recognizable stages of spermatogenesis when treated with deionized water for several hours. We investigated this problem by subjecting adults of P. gralli to various osmotic conditions and different temperatures to determine the most important factor in the tissue destruction. Worms with impaired testes were then transplanted to chickens to see if regrowth of the testicular tissue would take place and, if so, how fast. Gravid adult P. gralli (40-60 days old) were harvested from chickens, placed in small petri dishes containing deionized water at 3 C, and removed in groups of 5 at hourly intervals for 6 hr. A second trial repeated this procedure at 22 C. A third trial involved immersing flukes in 0.85% saline (NaCI) at 22 C. Worms for transplantation were kept in deionized water for 3 hr at 22 C and then placed in Hedon Fleig's saline (Dawes, 1954) and allowed to warm gradually to 39 C for 2 hr. These worms were then transplanted on polished glass rods in groups of 4 per eye directly to the orbits of uninfected chickens of the same age as the original hosts. The worms were then harvested at daily intervals up to 10 days. All resulting flukes were prepared for paraffin sectioning, stained with hematoxylin and eosin, and observed for histological abnormalities and evidence of the production of normal stages of spermatogenesis. The testes of worms exposed to deionized water at 3 C showed signs of vacuolization after 3 hr (Fig. 1). A progressive loss of the stages of spermatogenesis was noted in the 4-6-hr specimens (Fig. 2). The vitellaria and ovaries of these worms were unaffected by treatment with cold deionized water up to 6 hr. In those worms subjected to deionized water at 22 C, the disruption of the testes was much faster, and by 3 hr no stages of spermatogenesis remained and large vacuoles were observed (Fig. 3). Worms immersed in 0.85% saline at 22 C were normal in morphology of tissues for the 6-hr duration of the trial. This series of experi-