Nitrogen uptake rates were measured as a function of time following saturating additions (15 μMg-at N·−1) of 15N-labelid ammonium, urea, and nitrate to N-starved cultures of the picoflagellate Micromonas pusilla Butcher. Uptake rates were estimated from both the accumulation of 15N into the cells and the disappearance of nitrogen from the medium. Transient elevated (surge) uptake rates of NH4+ and urea were observed after enrichment. During the first 5 min the initial urea and NH4+ uptake rates were 2- and 4-fold greater than the maximum growth rate (μMmax)observed prior to No3− depletion in the cultures. The elevated urea uptake rates declined quickly to a relatively constant value, whereas the initial rates of NH4+ uptake declined rapidly but were followed by a subsequent increase prior to remaining roughly constant. Nitrate was not taken up as readily by N-starved M. pusilla as the reduced N forms. Although NO3+ uptake commenced immediately after enrichment (i.e. no lag period) the N-Specific rate over the next 6 h averaged half the μMmax observed during NO3− replete conditions.