Experimental infections were carried out with the tissue-dwelling filaria Dipetalonema viteae using the argasid tick Ornithodorus moubata as the intermediate and the multimammate rat Mastomys natalensis (Strain GRA Giessen) as the final host. The optimum infective dose was found to be 50 third-stage larvae, which produced patent infections and the recovery rates of adult parasites were 47.6 and 26.4% of the inoculated larvae 140 and 189 days after infection, respectively. After an average prepatent period of 57 days, the microfilaraemia increased progressively and reached relatively low maximum values about 192 days after infection. These maximum values were followed by rapid decrease of microfilaraemia, but microfilariae were still detectable at 261 days post infection. Following the subcutaneous injection of infected animals with dexamethasone in single doses each of 1, 10 or 20 mg/kg body weight 30 minutes before blood puncture, a dose-dependent increase in the microfilarial counts in the circulating blood was observed, this reaching maximum values between 120 and 160 days after infection. Repeated administration of single doses of 10 mg/kg dexamethasone revealed an uniform but temporary increase in the microfilaraemia but this was not associated with any alterations in the reproductive organs of adult female parasites. No correlation could be found between the number of microfilariae in the circulating blood and the number of adult worms recovered from the subcutaneous connective tissue. At necropsy 300 days after infection living female parasites could not be found any more.