WHILE engaged in the “in vitro” assessment of oxytoxic activity of posterior pituitary extracts, we observed a progressive decline in strength besides an increasing sluggishness of the responses of the young guinea pig’s uteri—in animals of the same stock and comparable body weights— to doses of a potent posterior pituitary extract. A careful checking of the experimental conditions showed that a very common panic-like green fodder (Brachiaria purpurascens (Raddi) Henry) had been substituted for the abitual kale leaves (Brassica oleracea L) of the ration. The grass, formerly intended as bedding, was eaten apparently as greedily as kale, together with the basic supplement of grain polishings always available “ad libitum.” Some normal, immature guinea pigs, besides the well known causes of variability in oxytoxic testing, like season, age, nutritional state, hormonal treatment or deprivation, are not suitable for oxytoxic testing. We shall try to evidence an unsuspected cause of variation of oxytoxic activit...