Fig. 11. Mean external contraction ratios in response to the membrane permeable cGMP analog, dibutyryl cGMP. The treatment demibranchs (numerator) were exposed to dibutryl cGMP and then to 2x10^-5 mol l^-1 5HT, while the control demibranchs (denominator) were only exposed to 2x10^-5 mol l^-1 5HT. Values are means ± 1 S.E.M., N=8 for each concentration of dibutryl cGMP.