The kinetics of calcium transport were studied in unmated (oestrous) and pregnant rabbits in the first half of gestation, with the aim of establishing evidence of hormonal (ovarian) influence on the pattern of transport. 2. The following tissues were examined at short- (45min and 2h) and long-duration (4, 16 and 48h) intervals after parenteral administration of (45)Ca or (47)Ca: maternal blood plasma, endometrium, uterine fluid, placental tissues, two developmentally disparate stages of rabbit conceptus, namely the unattached blastocyst and the early post-implantation foetus, and bone (femur). 3. Marked variability in calcium content characterized rabbit tissues and body fluids. 4. Compartmental analysis was applied to measurements of specific radioactivity. Oestrous endometrium had the largest rapidly exchanging calcium fraction (turnover time of 12min) and the highest value for calcium flux (500mug of Ca exchanged/h per g fresh wt. of tissue). A marked downward gradient in values of flux existed between the progestational endometrium, uterine fluid and blastocyst; there was a similar gradient between placental tissues and foetus. 5. An hormonal influence on calcium transport was evident in (i) the decrease in specific radioactivity of rabbit blood plasma with advancing pregnancy, (ii) the extraordinarily rapid calcium transport between blood plasma and endometrium, especially in the oestrous stage, and (iii) the effectiveness of ovarian hormone substitution in ovariectomized rabbits. 6. The very low specific radioactivity recorded for bone indicated that only a minute fraction of its calcium was exchanging with that of blood plasma under the experimental conditions examined. 7. The rate of uptake of (45)Ca by rabbit blastocysts growing in vitro was one-tenth of that of (22)Na, or that recorded for calcium in vivo. 8. Inhibition of carbonic anhydrase activity with acetazolamide in vivo, in maternal erythrocytes, endometrium and placental tissues, produced no appreciable changes in calcium uptake in these tissues or other systems examined as a routine on either day 6 or days 12-14 of gestation.
