Javascript Function Not working

function Shape()
{
    var lbl1=document.getElementById("<%= Label3.ClientID%>");   
    var ddshap=document.getElementById("<%= ddshpe.ClientID%>");
    if(ddshap.value=='Rectangular')
    { 
       lbl1.innerHTML='Length of sample in cm';
       document.getElementById("rectangular").style.display = '';
    }
    else
    {
       lbl1.innerHTML='Diameter of sample in cm';
       document.getElementById("rectangular").style.display = 'none';
    }
}

<asp:DropDownList ID="ddshpe" runat="server" Height="22px" onchange="Shape();" 
CssClass="Textbox">
<asp:ListItem>Rectangular</asp:ListItem>
<asp:ListItem>Cylindrical</asp:ListItem>
</asp:DropDownList>

protected void Page_Load(object sender, EventArgs e)
{
   ddshpe.Items.FindByText("Cylindrical").Selected = true;
}