Free and total cholesterol levels in the serum of 53 rhesus monkeys were determined. Large variations were observed among animals. Large fluctuations were also found in repeated weekly assays of serum cholesterol in many individual animals when reported in the conventional manner, i.e. in mg/100 ml of serum. The latter fluctuations tended to disappear when values were recalculated as fractions of dry serum solids. This finding indicates that the variability observed was in most instances due not to altered sterol metabolism but rather to changes In the water content of serum. The average total cholesterol values were 136.7 mg/100 ml serum and 123.0 mg/10 g serum solids. For free cholesterol the values were 37.8 mg/100 ml and 34.2 mg/10 g, respectively. Distribution of data according to sex, weight, time of day, end season of the year showed insignificant diurnal fluctuations and only slight variations due to the other variables.