to make textbox visible/invisble on selecting value from drop down list

Hi,

I have a drop down list with two values(eg. serial no,order no.) and have two text box(textbox1,textbox2). What i want is ,when you select a one value(serial no.) from drop down list your textbox1 must be visible and when you select order no. from drop down your textbox1 must become invisible and textbox 2 must be visible.(textbox1-serial no , textbox2- order no) and vice versa. How can i do this? Please help me.

Thanks and regards,

Sheetal Sable