Structure-activity relationships were investigated in a series of semirigid nicotinic agonists. Three of the agonists, (-)-ferruginine methiodide, arecoline methiodide, and its ketonic analogue arecolone methiodide, were cyclic analogues of anatoxin-a, a potent, naturally occurring, bicyclic alkaloid. Two other cyclic agonists, (-)-cytisine and (+/-)-muscarone, and the simplest agonist, the tetramethylammonium ion, were also tested. Arecolone methiodide and (-)-ferruginine methiodide have been tested as nicotinic agonists for the first time. Relative potency was assayed by contracture on the rectus abdominis muscle of the frog Rana pipiens. Natural, (+)-anatoxin-a, the most active of all of the agonists, was more than twice as potent as racemic anatoxin-a. Arecolone methiodide ranked after anatoxin-a in potency, being 8.6 times more potent than carbamycholine. A correlation between nicotinic potency and steric requirements probably involves the position of positively charged groups out of the plane defined by the carbonyl group and its two substituents. Channel properties induced by the agonists were evaluated by Fourier analysis of the end-plate current noise that resulted when the agonists were iontophoretically applied to frog sartorius muscle fibers. Average channel lifetimes were exponential functions of membrane potential, but the voltage sensitivity of channel lifetime seemed to vary among the agonists. Channel conductance, which was independent of membrane potential, also varied significantly among the agonists. The average charge traversing the membrane through each open channel, calculated from the product of average channel lifetime and current, did not correlate with potency. Therefore, the dominant component of potency is the frequency of channel opening. No clear relationship between the structure of the agonist and channel lifetime or conductance was evident.