Fig. 5. Dose–response curve for the effect of cyclic GMP on fluid secretion. Data are presented as means ± 1 S.E.M., N=5–11. The EC₅₀ value determined was 490 µmol l^–1 (r²=0.999, 95 % confidence intervals 99.7 µmol l^–1 to 2.4 mmol l^–1). Note that data for the highest cyclic GMP concentration have been excluded from the non-linear regression analysis because they indicate some receptor desensitisation at high levels, and this cannot be accommodated by the algorithm used.