1 - Dogs infected with Dirofilaria repens from Italy were found to show seasonal variations in the number of microfilariae in the peripheral blood. 2 - The rise in the number of microfilariae occured in August and September and is coincidental with the higher probability of trasmission. 3 - Rise and fall of microfilaraemia was induced experimentally by exposure of the host to high and low temperatures respectively. 4 - The importance of such variations in microfilaraemia is stressed in relation to the evaluation of the infection from a clinical, pharmacological and epizoological point of view.