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1. Investigations were made into the influence of the ionic environment on the steroidogenic response of the rabbit ovary to luteinizing hormone (LH). 2. Removal of Ca2+ from the medium was without effect on the response to LH. A similar result was obtained in Ca2+-free medium containing EGTA. 3. A tenfold increase in [Ca2+]o to 25.6 mM, or the addition of La3+ or Eu3+ (0.25 mM) to medium containing the normal concentration of Ca2+, caused a marked inhibition of the response to LH. 4. Removal of Na+ from the medium, and replacement by choline, had no effect on the response to LH. Replacement of Na+ by Li+ inhibited the response to the hormone strongly, but the addition of 4 mM-Li+ to normal medium was without effect. 5. Removal of K+ from the medium inhibited LH-induced steroidogenesis, whereas a twentyfold increase in [K+]o to 100 mM had no effect. The response to LH was also unaffected by the absence of Cl-. 6. Ouabain (10(-4) M) inhbited the response to LH, but nupercaine (10(-4) M) was without effect. 7. The inhibitory effect of ouabain was reversed by the addition of 2 mM-NADP+ to the medium. In contrast, the inhibitory effect of Eu3+ persisted in the NADP+-rich medium. 8. It is suggested that the intracellular ratio of Na+ or Li+) to K+ is important for the expression of the steroidogenic response of the ovary to LH. Altered concentrations of these ions might affect the formation or availability of NADP+. The inhibitory effects of high [Ca2+]o and lanthanide ions, however, are probably due to inhibition of hormone-stimulated adenyl cyclase.