SummaryThe anti‐Pneumocystiscariniiresponse of terbinafine together with that of three other compounds, trimethoprim sulphamethoxazole (TMP‐SMX), atovaquone (ATQ) and albendazole (ALB), has been investigated in immunosuppressed Sprague‐Dawley rats with established pneumocystosis. Drugs were administered orally (terbinafine in dosages of 40 and 80mg/kg per day, TMP 12·5 mg/kg per day plus SMX 62·5 mg/kg per day, ATQ 100 mg/kg per day and ALB 600 mg/kg per day) to six rat groups except one which served as a control, P.cariniipneumonia (PCP) was identified post‐mortem in nine (90%) of the control rats which exhibited a marked P.cariniiburden, and mean lung weights were higher with respect to the other treatment groups. During treatment, five rats in the control group died, whereas between 11 and 13 rats in all treatment groups survived. In the terbinafine groups (40 mg and 80 mg/kg per day), a mild P,cariniiinfection developed in three and two rats (27·2 and 18%), respectively, and almost the same infectivity score was obtained for those treated with 40 mg and 80 mg/kg per day. Histological changes in the lungs in animals receiving terbinafine treatment were minimal. Among the remaining compounds the rate of infection was seven (58·3%) for the ALB treatment group and five (45‐4%) for the ATQ group (mean score 19‐4 ±7‐1 and 23 ± 2·1, respectively). In the TMP‐SMX treatment group, there were 13 surviving rats and P.cariniiorganisms were found in two (15·3%, mean i