Martin (1956:389) stated that growth in young prairie voles was, in general, most rapid in the period April-May-June and least rapid in mid-winter. However, his data were based entirely on weights. The high incidence of pregnancy in the larger young females in spring and early summer may have caused the trend. Measurements taken by me of lengths do not bear out the idea of more rapid growth in the spring and summer, but, indeed, show the opposite. In most instances, voles of comparable sizes made significantly more rapid growth in the colder half of the year (mid-October to mid-March) than in the warmer half. Dividing the young voles in eight size groups and separating each group into comparable summer and winter samples, I found more rapid average growth in the summer sample in only two instances. These deviations from the general trend probably resulted from inadequately small sizes of some samples. On the average, the growth rate in summer was 92 per cent of that in winter. [Illustration: FIG. 5. Over-all length plotted against weight in young prairie voles, from newborn to the minimum size at breeding maturity. The range of variation increases as development proceeds, especially after the age of weaning is attained.] SIZE AND AGE AT SEXUAL MATURITY Greenwald (1956: 220) found that in females of _Microtus californicus_ some individuals are extremely precocious sexually, and might, at an age of as little as two weeks, produce corpora lutea and have sperm in the uterus. Greenwald mentioned one perforate female which weighed only 10 grams, but most reached a weight of at least 30 grams before their first pregnancies. The sterile cycles passed through earlier seemed to represent a "tuning-up" stage before establishment of the pituitary-gonad relationship. [Illustration: FIG. 6. Weight plotted against age in young voles, from birth up to 25 days. The range is wide at the start and increases as development proceeds.]