In this second series of studies on dirofilariasis in primates, 13 macaques were inoculated with infective larvae of Dirofilaria repens and were killed 2 weeks to 11 months postinoculation. Two of these animals were treated with prednisolone (as an immunosuppressant) daily beginning at 2 months of infection. Larvae and adult worms were recovered from 10 monkeys, but microfilaremia was not observed in the untreated monkeys despite the presence of gravid female worms. Eosinophilia and filarial antibodies were notable in monkeys harboring gravid worms, especially at 2, 4.5, and 6 months, probably corresponding to the period of final molt, mating, and microfilarial production, respectively. Microfilaremia appeared in the two prednisolene-treated monkeys at 5.5 and 6 months of infection, respectively, suggesting that under similar conditions infection with D. repens could produce a microfilaremia in man.