Abstract It is generally accepted that blood inorganic phosphorus (Pi) is higher in neonates than in adults. This study was conducted to determine the age or stage that serum Pi (SPi) in neonatal lambs falls to the level of that of an adult sheep. We also measured whole blood Pi (WBPi) to evaluate its value as a criterion in estimating the phos- phorus status of lambs. Eighty lambs (equal numbers of females and males) were selected at birth, and blood samples were taken from jugular veins at 13 stages: 15th, 30th, 60th, 90th, 120th (weaning), 155th, 190th, 225th, 260th, 295th, 330th, 365th, and 400th days of life. After weaning, the number of lambs decreased to 46. Calcium, magnesium, and packed-cell volume (PCV) levels were also determined. At the 225th day of life, serum testoster- one was estimated in two ram-lambs, and progesterone and estrogen were estimated in ten ewe-lambs, which showed negligible amounts. Sixty-seven ewes were sampled to determine their SPi status, and these data were considered as baseline reference value. There were significant differ- ences among the lambs’ SPi levels before and after weaning ( <0.01). Also, we observed that lambs’ SPi levels prior to weaning are significantly higher ( <0.01) than that of their mother ewes. In contrast, there was no significant difference between the lambs’ and ewes’ SPi levels after weaning. We suggest that one of the major causes of such a remarkable decline in lambs’ SPi after weaning may be due to the change from milk-based ration to completely dry ration, probably due to the considerable level of salivary Pi secretion.

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