The use of fecal steroid analysis to assess gonadal and adrenal function in primates has rapidly increased in recent years due to the ability to collect feces from nonhuman primates living in wild conditions. These techniques offer an exciting new potential for enhancing our knowledge of the endocrine status of free-living animals. Prior to using these techniques under field conditions, it is important to determine the diurnal variation of fecal excreted steroids for assessing possible time limitations on fecal collections. The following study investigates the diurnal frequency of defecation and patterns of steroid levels excreted in feces from four female common marmosets, Callithrix jacchus, living in a family group. These females represented three reproductive conditions: early pregnancy, ovarian cycling, and noncycling (postpubertal). Cortisol, estradiol, and progesterone were extracted and analyzed by enzyme immunoassay. Diurnal variations in steroid levels were found by ANOVA for cortisol and progesterone but not for estradiol. Significantly higher levels of cortisol were found in the afternoon, while the reverse was found for progesterone. All females showed the same pattern of steroid level change, except for cortisol in the pregnant female. Since all females defecated within the first hour after they awoke in the morning, this time was determined to be the most effective time to collect feces. The consistency of our findings reinforces the usefulness of this approach for studying reproductive and adrenocortical function in marmosets and also indicates that fecal collection should be limited to either morning or afternoon collections. Am. J. Primatol. 46:105–117, 1998. © 1998 Wiley-Liss, Inc.