During three rice-growing seasons in Uruguay, field experiments were conducted to study the contribution of cyanobacterial inoculation and chemical N fertilization to rice production. Neither grain yield nor fertilizer recovery by the plant were affected by inoculation with native cyanobacterial isolates. A low fertilizer use efficiency (around 20%) was observed when labelled (NH4)2SO4 was applied at sowing. Recovery of applied 15N by the soil—plant system was 50%. Inoculation did not modify 15N uptake by the plant when the fertilizer was three-split applied either. The total N-fertilizer recovery was higher when the fertilizer was split than when applied in a single dose. Plant N-fertilizer uptake was higher when the fertilizer was applied at tillering. Uptake of 15N from cyanobacteria by rice was studied in a greenhouse pots experiment without chemical nitrogen addition. Recovery of 15N from labelled cyanobacteria by rice in greenhouse growth conditions was similar to that of partial recovery of (NH4)2SO4 applied at sowing in the field. Cyanobacterial N mineralization under controlled conditions was fast as cyanobacterial N was detected in plants after 25 days. Moreover 40 days after inoculation non-planted and inoculated soil had more inorganic N than the non-inoculated one.