In the sea urchin (Hemicentrotus pulcherrimus) embryo, the arylsulfatase activity is known to increase dramatically after the mesenchyme blastula stage. In the present experiment, we have studied the mechanism underlying this increase in the arylsulfatase activity by using the inhibitors of transcription and translation. Inhibition of transcription by actionmycin D before hatching prevented the increase in the arylsulfatase activity, while inhibition of transcription after hatching failed to prevent the increase in the enzyme activity. Inhibition of translation by emetine always reduced the arylsulfatase activity. These results have been interpreted as that the arylsulfatase activity in the blastula embryo is supported by the pre-synthesized mRNA, and that the marked increase in the enzyme activity which is observed from the mesenchyme-blastula stage to the prism stage is due to the transcription of the arylsulfatase gene which occurs during the development from hatching to the mesenchyme-blastula stage.