The true rate of sterol synthesis in liver cells was determined by measurement of the weight of desmosterol produced over a given time period during incubations in the presence of triparanol. The simultaneous presence of tritiated water (3H2O) during the incubations permitted a direct observation of the weight of tritium incorporated into a given mass of newly synthesized sterol. The incorporation of tritium per atom of sterol carbon (H/C ratio) was lower than some previously reported values and suggests that a sizeable proportion of the reducing equivalents (NADPH) required for sterol synthesis arises via the pentose phosphate pathway. The H/C ratio changed significantly with length of the incubation period. The value of the ratio was also dependent upon whether the acetyl-CoA units utilized for sterol synthesis were derived predominantly from a carbohydrate or a fatty acid source.