The site(s) at which the gas-exchange region enlarges after alveoli are formed from the saccules of the immature lung is unclear; however, this information might be important to studies of the regulation of lung growth. Although aware of important assumptions on which it rests, we undertook this study to test the idea that the lung's gas-exchange region enlarges more rapidly in the immediate subpleural region than more centrally. To label the interstitium of the gas-exchange region, rats were provided silver-containing water from age 23 to 135 days (112 days). Some were killed at age 135 days, others 51 days after silver exposure ended (age 186 days). We considered silver grains that formed in the interstitium as a marker of lung present or formed during silver exposure; tissue added after exposure would diminish the numerical density of grains and the fastest growing sites would have the lowest grain density. Rats killed on the 112th day of silver exposure had a silver grain density in their immediate subpleural gas-exchange matrix that was 28% lower than in their more central gas-exchange matrix. Rats killed 51 days after silver exposure ended had a grain density in the immediate subpleural region that was 65% lower than in the central matrix of the gas-exchange region. The grain density in the rats killed 51 days after silver exposure ended was 33% lower in the central matrix and 67% lower in the peripheral matrix than in the respective regions of rats killed on day 112 of exposure.(ABSTRACT TRUNCATED AT 250 WORDS)