Rat peritoneal mast cells (MC) co-cultured on a monolayer of 3T3 fibroblasts (MC/3T3) were continuously exposed to compound 48/80 for 14 days. As early as 2 days following continuous exposure to compound 48/80, the MC/3T3 appeared as a heterogeneous population, with various MC appearing partially or fully degranulated or intact; this morphological pattern continued throughout the duration of the experiment. MC/3T3 remained functionally active as demonstrated by their ability to secrete histamine 15 min after each replacement with fresh medium containing compound 48/80, although this capacity diminished towards the end of the 14-day experiment. Concomitant with the histamine release, a significant increase in cellular histamine pools was observed. When MC/3T3 continuously exposed to compound 48/80 for 7 or 14 days were acutely challenged with anti-IgE antibodies, they were able to secrete histamine and prostaglandin D2 in amounts similar to those produced by control MC. In contrast, when these cells were challenged on day 7 or 14 with a higher dose of compound 48/80 or with substance P, the release of histamine was partially inhibited. Our results indicate that continuous in vitro exposure to compound 48/80, and the resulting MC degranulation product histamine, does not adversely affect the ability of MC/3T3 to synthesize histamine and to respond to activation stimuli of a related secretagogue for 7 days and a non-related one for at least 14 days.