The ability of oral tetracycline to inhibit the development of third-stage infective larvae (L3) of Brugia pahangi to adult worms in jirds was studied using 2 experimental protocols. Jirds treated with 1.4% tetracycline in drinking water for a period beginning 30 days before inoculation of L3 until 30 days post-inoculation (DPI) had 97% reduction in adult worm recovery compared to untreated controls. Jirds that received 1.2% tetracycline in drinking water beginning 1 day before until either 12 or 26 DPI had adult worm recoveries of 11% and< 1%, respectively. Untreated jirds and those given tetracycline beginning at or later than 13 DPI had similar adult worm recovery (27-29%). Prepatent periods were prolonged, and circulating microfilariae were reduced in jirds given tetracycline from 27 to 54 DPI compared to controls. These data indicate that tetracycline administered to jirds in drinking water inhibits B. pahangi development from L3 to adult worms and suggest that this effect occurs during early larval development. Tetracycline administered to infected jirds prior to and continuing through the onset of patency can also affect development of microfilaremia.