1 The rate of methaqualone metabolism in women was shown to be significantly increased at the time of ovulation. 2 The apparent first order rate constants for the formation of five C-monohydroxy metabolites of methaqualone on day 15 of the menstrual cycle were approximately double that on day 1. 3 The N-oxidation of methaqualone showed considerable inter-individual variation in its sensitivity to the menstrual cycle, and in a group of ten women the difference in N-oxide excretion between days 1 and 15 was not statistically significant. 4 The serum clearance of methaqualone on day 15 was higher (mean value 94.6 ml min-1 on day 1, 176.0 ml min-1 on day 15), serum half-life shorter (mean t1/2 beta 16.3 h on day 1, 11.6 h on day 15) and the AUC alpha smaller (mean value 44.0 micrograms ml-1 h on day 1, 24.4 micrograms ml-1 h on day 15) than on day 1. 5 The relative importance of the five hydroxy metabolites was unchanged during the menstrual cycle but the C/N oxidation ratio was greater on day 15 than on day 1. 6 The data for methaqualone metabolism in a control group of men was similar to than in women on day 1 of a menstrual cycle.