The postsynaptic actions of neurohumors on molluscan muscle may be exerted through control of force as well as by means of excitation and inhibition. The control of force may appear as potentiation of the response to excitation, as increased inotropism in spontaneous contractions, or as an increase in tonus. We have directed our attention to the alterations of force induced in aplysiid ventricles by applied postulated neurohumors. The results are interpreted in terms of the known effects of neurohumors on resting potential and action potentials of molluscan hearts. We recorded compound membrane potentials of aplysiid ventricles extracellularly, using a single sucrose gap apparatus together with a force-displacement transducer to measureforce in contractions or contracture. Aplysia californica ventricle has a membrane potential of −52.5 ± 9.4 mV. Ventricles of Aplysia dactylomela or Aplysia californica are depolarized by increased concentrations of external potassium ion, with an accompanying contracture. After incubation in calcium-free medium, KCl contracture-force is directly dependent on calcium ion concentration. A depolarization of 8.3 ±2.14 mV in potassium-free medium is blocked by substitution of lithium for sodium in the medium, suggesting an electrogenic sodium pump. There is a sustained depolarization in low chloride medium, which suggests a significant chloride contribution to the resting potential. Ventricles of Dolabella auriculana or Aplysia dactylomela are depolarized by acetylcholine (ACh). Threshold for depolarization is lower than threshold for contracture-force. The ventricle of A. dactylomela is depolarized by 5- hydroxytryptamine (5HT) with threshold at 10−9 M and a maximum depolarization of 30 mV at 10−4M. Depolarization by 5HT may induce beating but does not induce contracture. Ventricles of Aplysia californica are not depolarized by ACh although beating ventricles are inhibited, and a depolarized ventricle in a tonic contracture may be hyperpolarized and relaxed by low concentrations. The force of contraction of the ventricle of Dolabella auricularia is dependent on the duration of the plateau phase of the cardiac action potential. The plateau is lengthened by 5HT with an accompanying increase in force of beat, and shortened by ACh, with an accompanying decrease in force of beat. The action potential in the ventricle of Aplysia californica is not differentiated into spike and plateau phases, and neither ACh nor 5HT has any marked effect on the form of the action potential. Nevertheless, isolated ventricles of Dolabella auricularia, Aplysia dactylomela, and Aplysia californica are all excited by 5-hydroxytryptamine, with a threshold at about 10−M. Both spontaneous beating and excitation induced in A. californica ventricles by 5HT are blocked by lack of the sodium ion, which may be responsible for pacemaker potentials in molluscan hearts.