The inhibitory effect of the intravenous anaesthetic ketamine on CaCl2-induced contractions in the isolated K+-depolarized uterus of the rat in Ca2+-free medium was compared with that produced by papaverine, theophylline and the calcium entry blocker verapamil. Pre-incubation for 20 min with either ketamine (0.3 to 3 mM), papaverine (3 to 30 microM), theophylline (0.1 to 1 mM) or verapamil (3 to 30 nM) induced parallel, concentration-dependent rightward displacements of the dose-response curves to Ca2+ (0.04 to 22 mM). The antagonism was competitive, except that due to verapamil, the Schild plot for which yielded a slope which differed significantly from unity. The calculated pA2 values (+/- s.e.mean) were: ketamine 3.90 +/- 0.07; papaverine 5.55 +/- 0.05; theophylline 3.99 +/- 0.1 and verapamil 9.54 +/- 0.24. These drugs differed in their ability to relax the sustained contraction induced by Ca2+ (1 mM) in K+-depolarizing solution. Ketamine and verapamil relaxed the preparation in a concentration-dependent manner whereas theophylline and especially papaverine were less potent and induced only partial maximal relaxation. The t1/2 of the relaxant effect was significantly less for ketamine than for verapamil (5 and 22 min, respectively). Only ketamine produced a relaxation comparable to that obtained by washing the preparation with Ca2+-free solution (t1/2 = approx. 5 min). Prior exposure of the depolarized uterine strip to a low concentration of Ca2+ (0.22 mM) increased the potency of ketamine, but decreased that of papaverine and theophylline, in antagonizing Ca2+ induced contractions. In contrast, this procedure did not affect the potency of verapamil. 5 The inhibitory effects of these drugs, excluding those of verapamil, were completely reversed after washing the preparations with a high-potassium Ca2+-free solution, 3-5 times for about 30-60 min. 6 These experiments provide further evidence that the relaxant effect produced by ketamine on the rat isolated uterus is due to its ability to antagonize Ca2+ movements competitively and also show that there are marked differences between the nature of the relaxant effects of ketamine and those of papaverine, theophylline and verapamil.