NGSpice stepping a component value

Hi,
does anybody of you use ngspice?

My version of ngspice has been compiled from the cvs sources, Jan 26th,
2008, 
with mingwgcc 3.4.5 on WindowsXP.

I am looking for a way to vary a component value like the .step command in

pspice, but I cant find this in the ngspice do***entation.

e.g.
..param rval=1
R1 1 0 rval
..step param rval 30,50,5
or so doesn't work


Thanks for your help!
Mario