The activity of carboxylesterase and cholinesterase in plasma, liver and lung of young rats at different ages (5-31 days old) have been measured. The cholinesterase activity in the tissues of both sexes were almost constant during the development, and were similar to adult male activity. The carboxylesterase activity towards methyl butyrate and 4-nitrophenyl butyrate in plasma of both sexes increased from negligible (5 days old) to adult male value (31 days old). The carboxylesterase activities in liver increased markedly during the period, whereas the lung activities increased only slightly. The toxicity of soman was 6-7 fold higher in 5 days old rats compared to 30 days old rats. The decrease in toxicity correlated well with the increase in plasma carboxylesterase.