Rainwater concentrations of either ammonium or nitrate were sufficient to stimulate chlorophyll a (chl a) production in bioassay experiments using Gulf Stream surface water collected off North Carolina during the summer of 1991..Previous studies primarily examined inshore waters and did not address the impact of rainwater ammonium. An increase in chl a occurred within 1 d of the addition of synthetic rainwater (2 or 5 % rainwater, 98 or 95% seawater) containing up to 10 laM ammonium; this increase was followed by a decrease in chl a the following day. A similar response to nitrate addition (5 % addition of 20 gM nitrate rain) was observed. In separate experiments, natural rainwater having nitrate and ammonium concentrations less than those in the experimental synthetic rain yielded a greater chl a response than synthetic rain when added at similar dilutions (0.5 to 5.0% rain). The maximum dissolved inorganic nitrogen concentration in the enriched seawater in these bioassays was 1.8 ~tM; prior to enrichment the maximum was <0.4 gM. Bioassay experiments begun 2 d after a major storm event (sustained NE winds with gusts to 13 m s-1 and ca. 390 gmol m -2 inorganic nitrogen deposition from rain) showed a chl a increase in response to addition of natural rainwater, but not to synthetic rainwater with similar dissolved inorganic nitrogen concentration. These results suggest that phytoplankton stimulants, in addition to nitrate and ammonium, exist in natural rain but not in the synthetic rain used in these experiments.