Reproducibility of dose response curves for xorphanol at δ opioid receptors. This graph represents inter-experimental variability for dose response curves. Three independent experiments were carried out in duplicate. Varying concentrations of xorphanol were used to determine the potency and efficacy of xorphanol in inhibiting the effect of 5 μM forskolin in producing cAMP, as described under methods. Maximal cAMP levels were in the range of 400–1000 pmole/well. Data have been normalized as described under methods. Error bars represent standard error of the mean of duplicate measurements for each point.