The comparative effect of salt (NaCl, KCl) and osmotic (PEG 6000) stresses on germination and early seedling growth of alfalfa seedlings is monitored. Seeds are germinated in Petri dishes under different salinity (0 to 500 mM) or PEG (0 to 50%) treatments for 9 days. Final germination percentage, germination rate index, mean daily germination, water uptake, ability to recover germination after exposure to severe stress, seedling length, seedling dry weight and vigor index are measured. In order to understand the specific response of alfalfa to salt and osmotic stresses, several biochemical parameters are evaluated by determining the contents of organic solutes (proline, amino acids, soluble sugars) and inorganic solutes (Na+, K+, Cl-). The results obtained show that the two stresses at high concentration significantly decrease all attributes of germination and growth of seedlings while in the case of moderate stress this effect remains insignificant. These results, which reflect the tolerance of the Algerian cultivar, are confirmed by the high percentages of recovery obtained after exposure of the seeds to severe stress. The use of isotonic PEG solutions indicates that the effect of salinity occurs through both an osmotic and an ionic effect and that the latter effect is more inhibiting. In addition, alfalfa seedling growth is found to be more sensitive than seed germination. The obtained solute levels show that the osmoregulatory mechanisms involved under salt stress are induced primarily by ionic toxicity rather than osmotic toxicity.