Microfilariae of Dirofilaria immitis were recovered from an experimentally infected dog and inoculated intravenously into adult female BALB/c mice. Each mouse received 1 X 10(5) microfilariae. In one experiment mice were sublethally irradiated with gamma radiation 24 h prior to microfilarial inoculation; microfilariae were counted at 11.00 and 23.00 h 7, 14, 21, 28, 35 and 42 days after inoculation. A similar experiment was performed in nonirradiated mice to ensure that sublethal irradiation had not caused microfilarial periodicity. In a third experiment microfilaria numbers were determined in sublethally irradiated mice at 4-h intervals for 48 h beginning 14 days after microfilarial inoculation; microfilaria numbers at each bleeding were compared to microfilarial density at the same time in the donor dog. In a fourth experiment 2 groups of mice were irradiated, inoculated and bled as before; one group was acclimated to light from 07.00 h to 19.00 h, whereas the other group was acclimated to light from 19.00 h to 07.00 h. The microfilaremia was subperiodic, varying regularly with time in both the dog and mice. Microfilariae in the dog reached maximum numbers during light hours. In each experiment, regardless of actual time, microfilariae in mice reached maximum numbers during dark hours.