The effects of several organic nitrogen compounds on the germination of seeds and growth in vitro of the seedlings of the orchid Cattleya are described. Compounds related to the ornithine cycle, such as arginine, ornithine, and urea, were able to replace NH4NO3 in inducing rapid growth of the seedlings and to give additive growth effects with NH4NO3.~ Proline and γ-aminobutyric acid were also moderately good sources of nitrogen. Phenylalanine, tyrosine, citrulline, glutamic acid, aspartic acid, glutamine, asparagine, and phenylurea were unable to replace NH4NO3 and function as nitrogen sources for the seedlings. These compounds, however, did not inhibit utilization of NH4NO3 supplied simultaneously in the medium. Glycine, α-alanine, valine, α-aminobutyric acid, leucine, phenylglycine, hydroxyproline, canavanine, and thiourea were inhibitory to germination and subsequent growth of the seedlings. This inhibition was either not or only slightly overcome by NH4NO3.