Fig. 9. Mean external contraction ratios in response to an NO donor, DEANO. The treatment demibranchs (numerator) were exposed first to DEANO, and then to 2x10^-5 mol l^-1 5HT, while the control demibranchs (denominator) were only exposed to 2x10^-5 mol l^-1 5HT. The open circle represents data collected in July, when potentiation is absent. Values are means ± 1 S.E.M., N=3-15 at each concentration of DEANO.