Fig. 7. Effects of 6 mmoll^-1 Ba²⁺ on fluid secretion in serotonin-stimulated tubules. At t=20 min, Ba²⁺ was added to the Malpighian tubules in the experimental group (open circles) and an equal volume of saline was added to the control group (filled circles). Data are expressed as means ± S.E.M. The experimental and control tubules did not differ significantly; N=11 for the experimental tubules and N=10 for the controls.