A study using 72 beef cows and their calves was carried out over a one year period in Northern Ontario to examine the serum alkaline phosphatase (SAP) levels of the animals during the onset of hypomagnesaemia. No evidence was found of a fall in SAP corresponding to the dramatic decreases in serum magnesium. At the same time, other factors relating to SAP were considered. The relative SAP levels of the cows were found to be remarkably consistent with respect to time, but those of the calves less so. The SAP levels of the calves were considerably higher throughout than those of the cows, though the difference became smaller later in the season. Although the SAP levels of the calves did appear to be related to their growth patterns, there was a tendency for cows with higher SAP to have faster growing calves. Furthermore, changes in SAP prior to calving were negatively correlated with calf weight gain. There were fairly high correlations (about 0.4) between SAP and serum phosphorus for the calves but not for the cows.