Over fifteen years (1935–49) no consistent change was evident in March egg numbers or the winter egg production of pullets to the end of that month. In birds with single-day interclutch gaps, however, the average March egg number showed an upward trend which paralleled the inverse curve for mean egg weight except during the war years. This improving performance of birds in full production was accompanied by an increasing variance among them. The proportion of pausing birds also increased over the years studied, but less regularly, and may have resulted from environmental causes. Two such contrary and apparently independent trends within a flock or line of fowls raise difficulties in assessing genetic changes which are occurring when the annual production is considered as a whole.