Sex Steroids in Guinea Pig Fetuses after Sexual Differentiation of the Gonads

Concentrations of testosterone (T), dihydrotestosterone (DHT), estradiol-17�3 (E) and progesterone (P) were measured in plasma samples from individual guinea pig fetuses between Days 29-55 of gestation. Umbilical artery blood was collected in capillary tubes and steroid concentrations were measured by radioimmunoassay after chromatography. The T levels in the plasma of males were significantly higher on Day 35 than on Days 45 and 55 of gestation. Both T and DHT concentrations were significantly greater in males than in females on both Day 35 and Day 45. The E concentrations were below the sensitivity of the assay at all stages of development and no sex differences in P were found at any gestational age.