Summary. The pharmacokinetics of various non-steroidal anti-inflammatory drugs were determined to find dosage regimens by which drug concentrations known as active from human anti-inflammatory therapy could be reached and maintained in rabbits during continued administration. Based on the pharmacokinetics and si de\ x=req-\ effects of the different drugs, phenylbutazone was selected for the fertility experiments. Treatment of male rabbits with phenylbutazone for 9 consecutive days significantly reduced seminal concentrations of PGE-2 and PGF-2\g=a\and tended to increase ejaculate volumes, sperm motility, and fertility. These results indicate that, at least in rabbits, inhibition of PG synthesis by prolonged treatment with non-steroidal anti-inflammatory drugs does not impair male fertility. Instead, chronic treatment with the drugs at non-toxic doses may improve sperm quality and fertility.