Twenty-four clinically normal horses were castrated by routine methods. Peritoneal fluid was collected prior to castration and at 1, 3, 5, and 7 days postcastration. Peritoneal fluid was collected on days 9 and 11 if nucleated cell (NC) counts were still markedly elevated on day 7. Peritonitis, defined as NC counts > 10,000/μl, was evident in 15 horses following castration. Mean NC counts peaked on day 5 but were <10,000/μl for 74% of the horses by day 7, and 90% of the horses by day 9. One horse had a NC count > 60,000/μl on day 11 when sampling ended. Postcastration peritoneal fluid was obviously blood-tinged in 21 horses. Peak RBC counts occurred on day 3 but markedly decreased by day 5. Elevated peritoneal RBC counts correlated well with elevated NC counts (P < 0.001). Horses with peritonitis tended to have fever (P < 0.05). Other clinical signs of peritonitis were not apparent.