Infective larvae of subperiodic B. malayi from South Kalimantan (Borneo), Indonesia collected from laboratory-raised Ae. togoi mosquitoes after feeding on infected mongolian gerbils (Meriones unguiculatus) were inoculated subcutaneously into the groin areas of 15 SD and 36 LE rats. Blood was examined weekly by membrane filtration and thick smears starting 10 weeks post-infection. Microfilariae were found in 3 SD and 4 LE rats, the mf infection rate of 20% and 11% respectively. The prepatent period was significantly shorter in the SD rats (99-112 days) than those in the LE rats (110-153 days). The patent period was longer in the LE rats (208-703 days) than in the SD rats (236-543 days), and the mf density was similar (17.5 mf/20 c.mm blood against 16 mf/20 c.mm blood). At necropsy, 6 (3 female and 3 male) adult worms were recovered from 3 of 6 SD rats and 12 (9 female and 3 male) adult worms from 4 of 20 LE rats; all worms were found in the testes. The results of xenodiagnostic, histochemical staining and measuring spicules and protuberances, demonstrated clearly the difference between both species of Brugia. All dissected Ar. subalbatus mosquitoes exposed to B. pahangi became infected (100%), but none of those to subperiodic B. malayi were infected (0%). The mf of both species of Brugia in thick films stained with naphthol-AS-TR-phosphate showed that the excretory and anal pores of subperiodic B. malayi mf exhibited acid phosphatase activity and only a little activity was seen in other parts; while B. pahangi mf showed heavy diffuse acid phosphatase activity along the entire length of the body.(ABSTRACT TRUNCATED AT 250 WORDS)