Public Class Form1
Dim myString As String
Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
myString = Me.ComboBox1.SelectedItem
For x As Integer = 0 To Me.DataGridView1.RowCount - 1
Me.DataGridView1.Rows(x).Cells(0).Value = myString
Next
End Sub
End Class