METHODS Twenty-nine sheep foetuses of 75-144 days gestational age (term is 147 days) were delivered by Caesarean section under chloralose anaesthesia. Spiral strips 2-4 cm long were cut from the ductus arteriosus and other vessels, and were suspended in an isolated organ bath, with an isotonic frontal-writing lever, counterbalanced to exert 2 or 5 g tension. The contractions were recorded on a kymograph with a magnification of twenty-fold. In some instances isolated rings of blood vessel were suspended from glass hooks. Guinea-pigs near term were killed by a blow on the neck and exsanguination. Blood vessels were removed from 104 foetuses after death, and isolated rings were suspended from glass hooks in an organ bath. Contraction of the muscle moved a lever whose spindle was mounted in precision ball-races in order to reduce friction: the minimum torque required was 20 dyn. cm. The spindle was attached to a variable air condenser, and the change in capacitance was detected by a frequency-modulation system and displayed on a Cambridge Slow Recorder. The magnification was 60-100 times and was