Menarcheal age of 5546 Warsaw girls studied in 1976 was estimated by probit analysis as 12.73+/-0.03 SD 1.10 and of 7771 rural girls studied during 1976-1978 as 13.40+/-0.02 SD 1.20. The difference was 0.67+/-0.04. The difference between the groups from Warsaw and rural areas where fathers had only primary education was 0.57+/-0.07 year, while that between the groups where fathers had the highest education in both localities was only 0.31+/-0.09. When in both environments the groups were equated both for primary fathers' education and also number of children in the family, the differences in menarcheal age were 0.37+/-0.28, 0.49+/-0.11 and 0.47+/-0.14 in families with one, two and three children, respectively. In rural areas the effects of fathers' education on menarcheal age of daughters was much higher than in Warsaw. The secular trend from the mid-1960s to the mid-1970s was 0.25 years per decade in Warsaw and 0.64 years per decade in rural areas, suggesting a decrease of social differences in maturation rate of Polish girls. Menarcheal age is sensitive to even small differences in living conditions.