The of onset of acyclicity and the duration of persistent vaginal cornification (PVC), the predominant postcyclic anovulatory condition, were measured in five cohorts of virgin mice staggered across a 6-yr interval. The average age of cessation of cyclicity varied significantly among cohorts, ranging from 11 to 16 mo of age. Seventy-five percent of the mice entered PVC after ceasing to cycle, the remainder entered directly into a persistent diestrous condition. The duration of PVC, which averaged between 2 and 4 mo, varied markedly among individuals and was inversely related to the age of onset of acyclicity. Consequently, the variance associated with the cessation of cyclicity was about 4-fold greater than that associated with the cessation of PVC. These differing variances suggest that the factors governing ovulatory failure and PVC cessation may differ.