A previous study (Davis 1949) showed that the striking differences in size between Norway rats (Rattus norvegicus) living in the residential areas of Baltimore and rats living on a farm near Baltimore were largely phenotypical. Laboratory experiments proved that rats from both populations grew at the same rate and attained the same size when they were raised on the same diet. At that time, data on certain growth and reproductive characteristics were not available for the farm rats. Hence it is the purpose of this paper to compare certain growth and reproductive characteristics of the two populations. Such comparative studies of the productivity of mammalian populations are rare. Morton and Cheatum (1946) compared the reproduction of deer in northern and southern New York State. Most other studies of reproduction refer only to one population and hence are not comparative. Obviously an analysis of reproductive rates is fundamental to an understanding of population changes, dispersal, and mortality rates. The rats were captured during two years from June, 1947 to June, 1949. The city rats were obtained by trapping in residential areas of Baltimore. There the rats lived in yards, basements, and garages and fed upon garbage. The farm rats were captured on a horsebreeding farm about 16 miles north of Baltimore. There the rats lived under the buildings, in the stalls, and between the double walls of the horse barns and cow barn. They ate horse feed, manure, and an occasional dead rat or fowl. This is the deficient diet. The city rats came from stationary, increasing, or decreasing populations. These changes reflected environmental changes as well as the result of reductions from trapping. Seasonal population changes due to seasonal differences in the reproductive rate are not detectable with present census methods. The farm population during the first year averaged about 300 rats. The changes were due both to local changes in the environment and to seasonal changes in reproduction. During the second year the population increased to about 600. After capture, the rats were autopsied in the laboratory, and the data recorded on punch cards. The numerical totals in the tables do not all agree because data for a characteristic may be lacking. For example in Table I the weights of 402 male farm rats were recorded but the lengths of only 390; the unrecorded ones were due to inability to measure the rat for some reason. These differences are small and are random and hence do not introduce any systematic error. The city rats here described are not the same rats analyzed in the previous paper (Davis 1949).