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Embryos of the pondsnail Lymnaea stagnalis perform a tumbling or spinning motion that begins during the trochophore stage and continues throughout development. The rate of movement may be related to the delivery of oxygen to the embryo, and we hypothesized that 1) increasing oxygen tensions would result in a decline in movement, and that 2) reducing oxygen tension would result in an increase in behaviors. Using a constant flow delivery system, we administered gas-equilibrated artificial pondwater treatment oxygen regimes (20[degrees] C) to egg capsules containing embryos either at the trochophore/veliger stage, or at the later hippo stage of development: 1) 30 min normoxia (100% sat.) /30 min hyperoxia (>300% sat.) /30 min normoxia; 2) a stepwise hypoxic treatment (30 min normoxia; 30 min 50% sat.; 30 min. 20% sat.; 30 min. 0% sat; 60 min normoxia). Motion was recorded using digital video and was standardized as revolutions/10 sec. Analysis used a repeated measures design. There was no significant effect of hyperoxia treatment on either the trochophore/veliger stage or on the hippo stage. Embryonic movement rate doubled at hypoxic levels of 20% or less. On return to normoxia there was a consistent, virtual cessation of movement approx. 15 min after return to normoxia that lasted for on average 8 min before a slow resumption of behaviors. Some aspect of the behavior is not related simply to oxygen availability as hyperoxia did not result in notable behavioral modifications. Hypoxia results in an increase in a putative ventilatory behavior. The sudden cessation of behavior may be the result of a payment of an oxygen debt and a redirecting of resources resulting in behavioral modifications.