In 16 anesthetized piglets, the effects of 1-deamino-8-D-argininc-vasoprcssin (DDAVP) upon urinary flow rate, relative urinary osmolarity (U/Posmol), fractional water excretion (U/PIn), fractional urta excretion (U/Purea/U/PIn), renal adenosine 3',5'-cyclic monophosphate (c-AMP) formation rate, and medullary c-AMP content were studied during the first 6 days of life. Glomerular filtration rate (GFR) was 0.19 ± 0.09 ml/min/g kidney wt (means ± SD) on day 1 and increased to 0.51 ± 0.11 ml/min/g kidney wt on day 6. It was not affected by the application of 20 μg DDAVP/ kg body wt. On day 1, U/Posmol(1.81 ± 0.29), urinary flow rate (5.36 ± 1.49 μl/min/g kidney wt), U/PIn(31.67 ± 6.10) and U/ Purea/U/PIn(0.84 ± 0.12) were not affected by DDAVP application. From day 2 onward, the reponse of these variables to application of DDAVP increased progressively. On day 6, the following responses were observed: Urinary flow rate decreased from 13.85 ± 7.02 μl/min/g kidney wt before DDAVP to 5.07 ± 2.19 μl/min/ g kidney wt after DDAVP. U/Posmolraised from 1.24 ± 0.27 before DDAVP to 2.64 ± 0.17 after DDAVP. U/PInincreased from 36.85 ± 13.11 before DDAVP to 109.39 ± 22.00 after DDAVP. U/Purea/U/PIndecreased from 0.75 ± 0.05 before DDAVP to 0.55 ± 0.04 after DDAVP. These data indicate that the small solute permeability of the medullary collecting ducts becomes increasingly sensitive to antidiuretic hormone (ADII) during this period.The increase in responsiveness of the urinary concentrating system with age was paralleled by the effect of DDAVP on renal c-AMP formation rate. In the diuretic control state, no correlation between renal c-AMP formation rate and the number of days was observed. However, after the application of DDAVP, the nephrogenic fraction of renal c-AMP excretion rate increased from 10.58 ± 4.49 pmole/min/g kidney wt on day 1 to 21.61 ± 8.10 pmole/ min/g kidney wt on day 6. Similar results were observed with the medullary c-AMP content. In diuretic control animals which had not received DDAVP, the medullary c-AMP content ranged between 636 ± 257 pmole/g wet wt on day 1 and 622 ± 75 pmole/g wet wt on day 6. However, in animals which had been infused with DDAVP, medullary c-AMP content increased from 684 ± 274 pmole/g wet wt on day 1 to 1536 ± 316 pmole/g wet wt on day 6.Speculation The unresponsiveness of the medullary adenylatecyclase to ADII in the newborn kidney might restrict, in addition to other parameters, the renal concentrating ability during the neonatal period.