The surface of rat visceral yolk sacs (VYS) of intact, viable rat conceptuses were continuously monitored with a microfiberoptic sensor optimized for detection of the reduced pyridine nucleotides, NADH and NADPH. Model chemical toxins, cyanide and alloxan, were used and evaluated on the basis of their differential ability to modulate NAD(H)- and NADP(H)-dependent cellular pathways, respectively. Exposure with 2 mM sodium cyanide for 5 min caused a reversible fluorescence increase of 325 arbitrary fluorescence units (AFU) and 225 AFU on Gestational Days (GD) 10 and 11, respectively. Exposure with 40 mM alloxan for 5 min resulted in a fluorescence decrease of 170 and 120 AFU on GD 10 and 11, respectively. Glutathione (GSH) levels in the VYS, as determined by HPLC, showed a marked decrease from 27.3 +/- 2.1 to 2.9 +/- 0.4 pmol/mg protein, within the 5-min alloxan exposure period on GD 10. No decrease in GSH levels was noted for the same exposure duration on GD 11. A 2-hr pretreatment with 25 microM BCNU [(1,3 bis(2-chloroethyl)-1-nitrosourea], to inhibit glutathione disulfide reductase (GSSG-Rd), resulted in an elimination of the fluorescence decrease, but still led to a significant drop in GSH levels as seen on both days of gestation. These results are consistent with overall changes in intracellular pyridine nucleotide concentrations, where the relative amounts of NADPH increase significantly and disproportionately from GD 10 to 11. The net oxidation of NADPH, through GSSG-Rd activity, appears to be responsible for the alloxan-induced decrease in surface fluorescence. Conversely, the cyanide-induced fluorescence increases appear to be the result of NAD+ reduction, mediated through the inhibition of the terminal cytochrome oxidase in the electron transport chain.