1. Heat-treatment of Rhodnius adults for 14 or 15 days before feeding may either inhibit egg production during the first reproductive cycle at normal temperature, or cause a great reduction in the number of eggs deposited. In the latter case, oviposition is long delayed.
2. During the subsequent cycles commencement of oviposition and egg production seem to be normal.
3. If heat-treatment is performed immediately after feeding for a similar period, subsequent egg production at normal temperature is apparently not affected, while oviposition is slightly delayed. During subsequent cycles, however, the reproductive activity of the females is not markedly different from normal.
4. The majority of eggs produced under either type of treatment show progressive sterility over a period covering four reproductive cycles.
5. Allowing heat-treated females to copulate with freshly fed normal males results in a great increase in the fertility of the eggs produced. Egg-production, however, does not seem to be affected.
6. The significance of these results is discussed.