dataset programmatically update VS2005

[Bob]

Hi,
I have a dataset that is filled OK and used as the datasource for a
datagridview.
A mouse click on the datagridview picks the row.
When I attempt to alter one of the ItemArray[n] values the statement
executes but nothing happens to the value. i.e. Inspection of the ItemArray
when single stepping shows it hasn't changed.
What am I doing wrong?
Thanks
Bob
Code follows:
this.mdsBatchAssignable.Tables[0].Rows[hti.RowIndex].ItemArray[0] = 3;
//executes but no effect

this.mdsBatchAssignable.AcceptChanges(); //executes but no effect.