Comparison of some parameters of serum and uterine fluid of pregnant, viviparous sharks (Carcharhinus leucas) and serum of their near-term young.

I. In eight females, serum urea concentrations of 134-336 ml\l/l. were interpreted as indications of previous environmental salinities. 2. Fetal serum solute concentrations usually resembled those of the mother, but in two cases where fetal urea was higher, it was interpreted as a lag in urea reduction following movement from salt to fresh water. 3. Uterine fluid was similar to maternal serum in all parameters except for its very low protein content. 4. No flushing of the uteri with environmental water occurs. 5. Independent osmoregulation apparently does not occur before birth, although required mechanisms may be present. 6. The full range of urea tolerance is present before birth.