With the introduction of continuous culture of Plasmodium falciparum it has become possible to study the factors involved in gametocyte production in vitro and thus eliminate the uncontrollable in vivo variables of the host. The authors have developed a method for measuring quantitatively the rate of production of gametocytes at any time in such cultures. The method is based on an estimation of the percentage of ring forms that develop into stage II gametocytes.Using this approach, it was found that dilution of cultures with fresh red blood cells so as to lower the parasitaemia led to rapid fall in the rate of conversion to gametocytes. The conversion rates subsequently rose again to levels typically in the order of 10% after several days of growth in the new culture. In the parental cultures from which the dilutions were made, conversion rates remained high at all times. This pattern was consistently observed in three different isolates of P. falciparum from Africa and the results indicate that the reduction of parasitaemia by addition of fresh cells was responsible for reducing production of gametocytes and that conditions associated with a period of growth in culture induced renewed gametocytogenesis. The authors conclude, therefore, that environmental conditions directly modulate the rate of gametocyte production by P. falciparum in culture.After 1(1/2) years in culture, parasites have retained their ability to produce gametocytes and the gametocytes to undergo exflagellation.