Cholesterol absorption was studied in groups of guinea pigs fed diets containing 0, 0.1%, or 1% cholesterol. A similar proportion of tracer cholesterol was absorbed regardless of the cholesterol content of the diet. Furthermore, the proportion of tracer cholesterol absorbed by individual animals did not change when the cholesterol-free diet was changed to one containing 1% cholesterol. Cholesterol absorption was also measured in hyporesponding guinea pigs. These guinea pigs had been fed 1% cholesterol-containing diets for nearly a year with minimal pathological effects. These hyporesponders had a decreased intestinal transit time, which enabled them to decrease the fractional absorption of cholesterol below the levels seen in the controls, and to absorb less cholesterol/kg body weight than the hyperresponders. Excretion of total and of neutral steroids was measured in guinea pigs fed 0 or 1% cholesterol-containing diets. The 1% cholesterol-fed guinea pigs increased the excretion of steroids 3-fold over control levels. However, they absorbed more dietary cholesterol than they excreted in any form. It seems, therefore, that a major cause of the cholesterol pool expansion in the guinea pig is its inability to limit absorption of dietary cholesterol in conjunction with its inability to sufficiently increase excretion of steroids.