Abstract Extract The clinical condition of “Kopziekte” or “grass tetany” in cows was recognized in the Netherlands for many years before Sjollema (1928, 1932) and his colleague (Sjollema and Seekles, 1929) first reported its association with low serum magnesium values in distinction to the low calcium of “milk fever”. The first parallel British observations were made by Dryerre (1932) who established pronounced hypomagnesaemia in cases of so-called “lactation tetany” in grazing cows. This term was introduced by Lothian (1931) who described the clinical effects without correlation of the blood findings. In cows recently turned out from stall to grass, Blakemore and Stewart (1933) found hypomagnesaemia not only in cases of clinical tetany but also in a proportion of the apparently normal cows of the same herd at the same time. Concurrently Hopkirk et al. (1933) reported a similar observation in cows at grass throughout the year in New Zealand.