A series of selected cyclotetrasiloxanes were synthesized by David E. Spielvogel and J. Franklin Kyde of the Corporate Chemical Research Department of Dow Corning. These compounds were derivatives pjhenylheptamethylk cyclotetrasiloxane or 2,6-cia-diphenylhexamet hylcyclotetrasiloxane; therefore, their antigonadotropic activity was evaluated in the male rat by the Biomedical Research Laboratory. The compounds were administured orally for a seven-day period. A few selected compounds were also administered subcutaneously for comparison. Derivatives considerably more potent thanthe above two reference-standard compounds were not found. However, morepolar derivatives based on anticipated metabolic biotransformation of the reference compounds were generally as active as the original reference compounds. These more polar derivatives were also very active subcutaneously unlike the original reference compounds.