A quantitative estimate of the role of calcium ions on the lipid--water partition coefficients of prostaglandin E1 and dinoprost suggested the possibility of prostaglandin molecules and calcium ions aggregating in a 14:1 ratio to produce a lipid-soluble aggregate. The aggregation is postulated to be a characteristic of prostaglandin molecules as compared to simple fatty acids, e.g., 1-octanoic acid, which, in the presence of calcium, behave differently than the prostaglandins. The uptake of prostaglandins by the mucosal surface of the rabbit intestine increased in the presence of calcium. For example, at 25 mM calcium, prostaglandin E1 was transported at approximately twice the rate as in the system containing no calcium. The uptake rate of dinoprost was estimated to be three times faster with 10 mM calcium than in the absence of calcium. Therefore, it is proposed that a carrier-mediated diffusion process, for both the prostaglandin molecules and calciumi ions, takes place in the uptake mechanism. Diffusion coefficients ranging from 0.48 X 10(-5) to 7.19 X 10(-5) cm2/sec and permeability coefficients ranging from 1.04 X 10(-2) to 15.6 X 10(-2) cm/sec were estimated for all systems studied.