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We examined the effect of cardioactive drugs on the heart rate of intact rat embryos at day 11 (25 somites) of gestation, the stage of development prior to cardiac innervation. Tested were the autonomic drugs methoxamine and isoproterenol, the β‐adrenergic receptor blocker propranolol, and the phosphodiesterase inhibitor theophylline. Our observations indicated that (1) methoxamine had no effect; (2) isoproterenol and theophylline caused in creases in the heart rate; and (3) propranolol inhibited the effect of isoproterenol but did not affect the response to theophylline. We concluded that our results are consistent with (1) Ahlquist's ('48) original concept of α and β adrenergic receptors and the observation that the heart has no α receptors, (2) with the role of cyclic‐AMP in the model of the adrenergic receptor proposed by Sutherland ('68), and (3) with the observations that the embryonic heart at this stage of development is not yet innervated and so is without the vagal reflex arc (Hogg, '57; Gomez, '58).

To study directly the effects of hypoxia on the mammalian embryo, rat embryos at the 14-, 22-, and 35-somite stages were explanted into the in vitro system of New. The initial heart rates in the three groups did not differ. With hypoxia the 22-and 35-somite embryos responded with a prompt heart-rate drop but no significant effect was found in the 14-somite embryos. Irreversible sludging of yolksac circulation was associated with death in the older embryos.