Mature gametocytes of Plasmodium yoelii nigeriensis were incubated in vitro with a variety of antibiotics whose activities were directed against DNA, RNA, protein and microtubule assembly. Following incubation for periods between 5 and 90 min, exflagellation was induced and the degree of inhibition produced by each drug was assayed microscopically. The results obtained show that de novo synthesis of axonemes and proteins is essential to the events of microgametogenesis. The inhibitory effect of actinomycin-D suggests that de novo RNA synthesis is also required. However, the inactivity of 8-azaguanine, a drug also effective against m-RNA metabolism, casts some doubt on the requirement for m-RNA synthesis. DNA synthesis is not required during the three successive mitotic division of the parasite genome during microgametogenesis. The mature microgametocyte therefore contains the 8c DNA required for microgamete production.