The possible mechanism of androgenization has been studied by intrahypothalamic application of actinomycin-D (0.17 µg), chloramphenicol (16 µg), cycloheximide (0.7 or 14 µg), puromycin (1.4 µg), rifampicin (15 µg), sarkomycin (12 µg), streptomycin sulfate (18 µg), atropine (11 µg) or procaine (15 µg), in the 5-day-old female rat. Implantation of these agents in cocoa butter pellets by itself did not disturb hypothalamic development, nor did these agents prevent androgenization, other than in one case. As the exception, cycloheximide when implanted into the suprachiasmatic region significantly attenuated the action of 30 µg testosterone propionate (TP) injected s.c. as evaluated by ovarian morphology at 45 days of age. Cycloheximide was not effective s.c. but two agents, sarkomycin and rifampicin, that were ineffective in the brain inhibited androgenization when given s.c. This inhibition may be related to the peripheral handling of TP. The effectiveness of intrahypothalamic cycloheximide against androgenization suggests than androgen may act through protein synthesis in the neonatal rat brain.