Yucca schidigera Roezl ex Ortgies, family Lillaceae, was fractionated with butan-1-ol to yield a butanol extractable fraction (BE; saponin fraction) and a non-butanol fraction (NBE; non-saponin fraction). Four groups of eight male rats were allowed ad libitum access to diets supplemented with water (control) or 200 mg kg(-)(1) total Y. schidigera (TOT) or 200 mg kg(-)(1) of each of the fractions (NBE or BE). The effects of dietary supplementation with the fractions and their interactions in TOT were analyzed according to the factorial experimental design by two-way analysis of variance. All three supplementation groups displayed significantly reduced serum urea levels (P < 0.05). The TOT and NBE fractions were found to significantly increase serum insulin levels (P < 0.01) in the absence of any fluctuations in serum glucose levels. Urea cycle enzyme activities, namely, arginase (EC 188.8.131.52) and argininosuccinate lyase (EC 184.108.40.206), were significantly decreased (P < 0.05) in vivo, although no effect was observed in vitro. Both fractions displayed effects, indicating that the active constituents are present in both fractions.