(I) Solutions of labelled ions 32PO4
2-, 24Na1+, 42K1+ and 131I,1- were administered parenterally to pregnant rabbits on Days o to 12 of gestation (the day of mating being designated o). Several experiments were also done with adult oestrous rabbits. Measurements of radioactivity were made at 45 min after ion injection. Results were expressed as the proportion, X IO-6, of the total radioactive ion injected, found present per mg tissue or fluid, fresh weight. (2) The following findings were made in normal, pregnant rabbits: (a) in the free-lying 6-day blastocyst values for all ions were low, that for PO4
3- being lowest, and, moreover, they showed a marked degree of divergence; this contrasted with the much higher and closely grouped values in the I2day foetus; (b) in the fluid withdrawn from the blastocoelic cavity on Days 7 to IO values for PO4
2- and Na1+ rose to a peak on Day 8, and declined on Days 9 and IO; (c) in the endometrial secretion all ion values greatly exceeded those in the free-lying blastocysts and in the blastocyst fluid, and remained relatively stable during Days 6 to 12 of pregnancy; (d) placental tissue retained more than twice the amount of PO4
3- found in the inter-implantation segments of the endometrium, a phenomenon absent or much less prominent with the other ions; (e) all ions except K1+ showed higher values in the endometrium and secretion of non-pregnant rabbits than in the progestational phase of pregnancy; (f) exceptionally high PO4
3- values were typical of the endometrium and secretion on Days 2 and 3 of gestation. (3) The incorporation of 32PO4
3- was studied more extensively than that of the other ions, in pregnant rabbits treated with oestrogens (stilboestrol, oestradiol benzoate), certain purine analogues (6-mercaptopurine, 8-azaguanine) and trypan blue.
In pregnant rabbits which had received single doses of stilboestrol or oestradiol benzoate 36 hr before 32PO4
3- injection, there was a depression in PO4
3- uptake on Days 3 and 4 in the endometrium and secretion; this was in disparity with the enhanced ion uptake evident from Day 5 onwards. In placental tissue a rise above normal occurred only on Day 7, and was followed by a progressive decline from Day 8 onwards, being most marked in the foetal placenta at 12 days. Retention of PO4
3- was distinctly increased in the 6-day blastocysts; on the other hand, very low values were recorded in 12-day foetuses. In the blastocyst fluid the peak at 8 days was obliterated owing to higher values on Days 7, 9 and 10.
Suitably timed treatment of pregnant rabbits with mercaptopurine, azaguanine, or trypan blue, produced no changes in ion incorporation in the endometrium or secretion. Values for the 6-day blastocyst remained unaltered. A small but distinct depression in ion retention in the 12-day foetus and placenta followed treatment alike with the purine analogues and trypan blue. (4) In the cervix or vagina no changes were observed in ion uptake analogous to those recorded for endometrial mucosa and secretion. (5) The significance of this experimental approach to the problem of blastocyst-uterine relationships is discussed with respect to the functional organization in the pre-implantation blastocyst, the diurnal shifts in the uterine environment, the differential response to oestrogen treatment of uterine tissues depending upon their nature and the period of gestation, and the impaired ability of damaged foetus or placental tissue to retain ions entering from the maternal bloodstream.