Daily early morning urine samples from 23 women during their conception cycles were evaluated for urinary estrone (E1) conjugates and pregnanediol-3-glucuronide (PdG) levels and indexed by creatinine (Cr) concentrations. These 23 cycles were classified into two groups according to the rate of E1 conjugates increase following implantation. Urine from 16 nonconception cycles was available for 11 of the women. The two groups differed significantly both before and after, but not during, the midcycle E1 conjugates peak. The length of the follicular phase was significantly shorter in the group of women with the lower E1 conjugate levels. These data suggest that differences in estrogen excretion at times other than the preovulatory estrogen surge represent the endocrine activity of subdominant ovarian follicles. Further, such urinary estrogen profiles may represent useful parameters for defining ovarian follicular populations and/or indicating the effects of hazards to female reproduction.