A comparison was made of contractions produced by submaximal doses of oxytocin, noradrenaline, PGE2 and PGF2 alpha in estrogen-dominated rat uterus after the preparation had been loaded in Ca-free medium supplemented with EDTA 3 mM. The experiments were carried out in the presence of EDTA 1 mM to complex the contaminating Ca. The contraction was sustained as long as the preparation was exposed to the drug and was relaxed by washing. Cumulative concentration-response curves to oxytocin (6.25-100 microM), noradrenaline (0.05-1.6 mM), PGE2 (0.1-1.6 microM) and PGF2 alpha (0.02-0.32 microM) were made. The threshold concentration for PGF2 alpha was much lower than for PGE2, oxytocin and noradrenaline. Isoprenaline (10- -10(-4)M), KCl (56.3 mM) and caffeine (5 mM) were added. The results showed that isoprenaline and KCl did not produce contractile response. Caffeine produces only a small decrease in the resting tension and this effect is not reversible. After addition of noradrenaline, a concentration of oxytocin (6 microM) produced a uterine contraction smaller than the control response of uterus to oxytocin. The response to the oxytocin applied after washing out the caffeine was the same as the control response. All agonists tested that were capable of inducing uterine contraction in Ca-free medium act through specific receptors. This suggests a relation between receptor-operated Ca-channels and intracellular Ca-stores.