The agonist activities of a range of prostaglandin analogues on smooth muscle preparations sensitive to prostaglandin E2 (PGE2) have been investigated. When necessary thromboxane-like activity was eliminated using the thromboxane receptor antagonists EP 045 and EP 092. On the bullock iris sphincter, rat stomach fundus and guinea-pig trachea, (+/-) omega-tetranor-16-p-chlorophenoxy PGE2 (ICI 80205) and 16,16-dimethyl PGE2 were more active contractile agents than PGE2, whereas for relaxant activity on the cat trachea, guinea-pig trachea and dog hind limb arterial vessels in vivo the order of potency was reversed. 11-Deoxy PGE1 exhibited greater relaxant than contractile activity when compared to PGE2. Iloprost and 6a-carba-delta 6,6aPGI1 (potent mimetics of PGI2) showed high contractile activity on the PGE-sensitive preparations. PGI2 was less active and another potent PGI2 mimetic, ZK 96480, showed only very weak activity. When tested, the dibenzoxazepines SC 19220 and SC 25191 blocked the contractile actions of iloprost and 6a-carba-delta 6,6aPGI1 and those of PGE2 and 16,16-dimethyl PGE2 to similar extents. Each of the PGI2 analogues showed weak activity on the relaxant systems. On the proximal portion of the ascending colon of the rat, PGI2, iloprost, 6a-carba-delta 6,6aPGI1 and ZK 96480 always inhibited spontaneous activity at nanomolar concentrations. PGE2 and PGE1 showed weak contractile activity. The distal portion of the ascending colon was more responsive to the contractile action of PGE analogues: both iloprost and 6a-carba-delta 6,6aPGI1 showed evidence of contractile activity, whereas PGI2 and ZK 96480 always inhibited spontaneous activity. Evidence was obtained that the rat stomach fundus also contains a PGF receptor; (+/-) omega-tetranor-16-m-trifluoromethylphenoxy PGF2 alpha (ICI 81008) acted as a specific agonist. PGF2 alpha and its omega-tetranor-16-p-fluorophenoxy analogue produced a higher maximum response that ICI 81008 probably due to their additional agonist action at the PGE receptor. The data support the hypothesis that there are two subtypes of the PGE receptor. ZK 96480 has minimal activity on both receptor subtypes and appears to be a highly specific PGI2 mimetic.