Minima in the total cross sections of nitrogen and oxygen have been investigated by measuring the spectra of uncollided fission neutrons transmitted through thick samples of the elements and by comparing the results with calculated spectra based on total cross sections only. Good agreement between the measurements and calculations was obtained for oxygen when the cross sections evaluated by Slaggie and Reynolds were used and for nitrogen when the cross sections measured by Foster and Glasgow were used. Calculations with other evaluated cross sections for these elements resulted in significant differences and indicate a lack of detailed knowledge of the valleys in the cross-section data. Measurements made for carbon and lead were in good agreement with calculations, as was expected since the cross sections for these elements are well known. The technique used for the spectral measurements employed an NE-213 proton-recoil spectrometer of high sensitivity.