The content and concentration of testosterone in fetal gonads of domestic pigs was investigated for the early stages of pregnancy. Measurements were made with a competitive-protein binding radioassay after column chromatography of the extracts. Gonads were obtained from 283 male fetuses ranging in size from 2.0 to 12.0 cm crown-rump (C-R) length. Testosterone was present in measurable amounts in gonads from the smallest specimens and reached a peak content of >4.0 ng per pair of gonads near the 4.0 cm C-R stage. A decrease in testicular testosterone values was then noted until the 5.1-5.3 cm C-R length and was followed by rising amounts of testosterone as the testes weight increased in the larger fetuses. Testes weight of 240 fetuses from 3.0 to 12.0 cm C-R length showed an almost linear increase over the entire range of fetal sizes. Gonads from 43 individual female fetuses (3.0-7.0 cm C-R length) contained no measurable amounts of testosterone. No testosterone was detectable in extracts of pairs of adrenal glands from 17 male and female fetuses (3.0-6.0 cm C-R length). These data suggest that fetal pig testes alone show a pattern of higher testosterone production at male sexual differentiation.