Experimental filarial infections of Mastomys natalensis, strain GRA Giessen, with Litomosoides carinii, Dipetalonema viteae, Brugia malayi (subperiodic), and Brugia pahangi were compared. Mean prepatent periods of 52, 57, 107, and 73 days p.i. were observed after subcutaneous inoculation of 40, 50, 85, and 70 infective larvae of L. carinii, D. viteae, B. malayi, and B. pahangi, respectively, in the neck region. All of the L. Carinii, D. viteae, and B. pahangi infected Mastomys showed a regularly detectable microfilaraemia. In B. malayi infections 95.5% of the animals developed parasitaemias, when the larvae had been inoculated in the neck region, whereas after groin infections only in 66.7% of the animals became patient. For both Brugia species, infections in the groin resulted in considerably lower microfilarial levels. Maximum microfilariae densities could be detected at day 120 (L. carinii) and at day 1980 (D. viteae) p.i. In the case of Brugia neck infections, the microfilarial levels increased usually until the end of the observation period, 300-350 days p.i. Worm recovery rates were 63% (L. carinii), 20.6% (D. viteae), 21.1% (B. malayi), and 31.4% (B. pahangi) of the inoculated larvae. When third stage larvae of Brugia species were inoculated in the neck region, adults of B. malayi and B. pahangi were isolated predominantly from the heart of lungs (84.4 and 78.5%, respectively). Only 12.3% of B. pahangi parasites were found in the testes; 3.4% and 18.1% were localized in the lymphatics. After inoculation of infective larvae in the groin more worms could be recovered in the testes and lymphatics, i.e. 23.4% and 14.9% (B. malayi) or 19.1% and 45.2% (B.pahangi), respectively. The results are discussed under the aspect of chemotherapeutic investigations for the evaluation of microfilaricidal, macrofilaricidal or chemoprophylactic compounds. It is concluded, that Mastomys natalensis, an animal with a broad spectrum of susceptibility for filarial infections, can be used as an alternative experimental model system, similar to that of the jird.