Fig. 4. for^s tubules display increased basal cGMP-phosphodiesterase (cG-PDE) activity. cG-PDE activity was assayed in preparations from head (open bars) and tubule (grey bars) from for^R and for^s animals as described. To aid comparison between head and tubule samples, cG-PDE activity in for^R is taken as 100%, and for^s activity normalised against this, for each tissue (means ± S.E.M., N=4-6), ^*P<0.05. for^R cG-PDE activities: head, 211±23 pmol min^-1 mg^-1; tubule, 642±72 pmol min^-1 mg^-1 protein.